It’s really a matter of optics.
Unless the line is so long you’re actually able to increase the size of your family by completing the adoption process during your out-of-queue antics then you’re not actually making it any longer.
ETA: But I stand by my earlier take: one person holding the place in line for a mob isn’t cool.
All of this, well said!
Great question! I think the answer depends.
I agree it’s usually a matter of optics. But optics are important. For example, if you’re waiting in a Quick Service line and a whole family joins the Dad in front of you just as he’s about to order, it can be frustrating, even though Dad was still going to order the same exact things whether his family was with him or not. That doesn’t mean it’s not rude. Lots of things that are rude are about how it makes the other person feel, not how it directly impacts their physical well-being.
Perhaps most importantly is the way in which you do the thing:
A mob crushing through a queue saying, “Make way, we’re more important than you!” (whether they say those words or just communicate them) … Very annoying.
A mom with her toddler saying, “I’m sorry, can I sneak past you? I had to change his diaper.” … most people are going to be very gracious in that situation. (It also helps if you tell the person next to you what you’re doing before you leave the line.)
General rules: the fewer people leaving the line, the younger the child, the less disruptive the reunion, and the further back in the line it occurs, the more polite the person is, the more likely it is to be acceptable.
My daugher (11) was with a group at Hersey park and they seperated from her. She went through the line trying to catch up with them and one person said, “absolutely not”.
I have told her many, many times that this woman had every right to do that. What my daughter remebers was the group running off without her, and that they would leave her once off the ride if she didnt catch up, and her very strong anxiety about this.
I will always remeber this when I see people, especially tweens cutting in line. But that doesnt mean I won’t still be annoyed! I am always fairly forgiving of one parent with one toddler.
I am not attacking the OP. Thank you for asking rather than assuming that it’s okay and everyone does this. This subject pushed all the right buttons for me!
Earlier this month at DLR, the brother told me that one parent would stand in line while the other parent went to do other things with the preschooler and joined the line later. He even did that with another family: one person wait in line and 6 people join later. 
He said everyone does that! I looked at him in the eye and said something like, “I think not. That is rude. I’ve been on multitude of rides the past 2 days at DLR. I think I have had one group join their party ahead of me. If everyone did this, there would be people constantly pushing their way to the front.” When people say everyone, it catches my attention! 
The queues are, for the most part, really too narrow for people to be constantly joining their groups in line later. I feel it’s okay to join the line while the queue is still outside and the rest of your party can join without pushing through people. I’m not okay with people joining later in meet & greet lines. More often than not, each person has to greet every character when they arrive and leave; and there tends to be multiple combination of groupings to take pictures.
But please take your child to the restroom, if he or she needs to go while in line!
Yes, this helps, I think. When it was just DW and me in 2020, we RDed FOP but weren’t at the front of the pack so we were somewhere along the path to FOTLK, and there was quickly a huge crowd behind us too. We both realized we needed to use the bathroom since we skipped that in the rush to get there, so I mentioned to the person behind me, “We’re just going to take turns using the nearby bathroom.” and I think the person just nodded, and it wasn’t a big deal as we were both back from our turns before the actual winding and narrowing queue, and there was no disruption by it.
I’d think minor things like that are ok and would be found acceptable by most people, but when it starts to become an intentional creation of a personal VQ where one is not supposed to exist, it’s not a good look.
This wouldn’t be frustrating to me at all. Since, as you said, Dad is ordering the same exact things regardless of it the other insert number here people are there or not. It doesn’t make a difference to me if they show up at the start of the QS line or at the table or anywhere in between.
You consider this to be rude?
Though, I guess if the order wasn’t known ahead of time- Once again, optics. If dad already knew the order and was prepared to give it, it wouldn’t make a difference when the family joins. But I guess if they join at the last minute and they’re hemming and hawing over what to get, that would definitely be obnoxious.
Just to offer an alternate perspective- maybe you will be pleasantly surprised in how your little one does in lines! I went with my oldest when he was about 2.5. He’s always been a big people watcher so between the stuff in the queue to pay attention to, and all the other people and children around, he did remarkably well. I remember the jungle cruise line sucking because he kept going under the ropes to other lanes but that queue sucks for adults, too!
I also prepped him by saying there will be lots of waiting before we get to ride to try and set expectations. Now all this is famous last words since I’m going back in May with him and his younger bro so a 5 yr old and 2.5 yr old but here’s hoping! 
Again, I think how it’s done is more important. The person behind them in line was thinking “oh this guy is going to order a dole whip and be done.” They’re mentally prepared for that. Then suddenly they realize that they’re behind a much longer line than they thought - multiple items, possibly complex, etc. It’s the same reason those switchbacks on Jungle Cruise queue are so annoying.
There are ways to soften the blow, like don’t send your whole party to meet dad - just send one runner to help him carry food.
This bothers me zero. One person keeping a toddler entertained, coming back into line before the inside where you aren’t really going past me. Totally fine to me. You didn’t make my wait any longer than it would have been if you were waiting in line. And it may have been more enjoyable without your kid pitching a fit. (Not that yours would but just saying.)
I am actually not a fan of rider swap. That does make my wait longer (depending on the ride of course) because now some parents bring back an older child for a second ride. So my wait just got increased by one rider. I get adding one more is allowed, so it’s within the rules but personally it bothers me more.
You keeping your toddler entertained doesn’t change my life, and may make it a little better.
I would say if it was me and a Mom and toddler had to run to the bathroom, I would not be bothered. Huge group is totally different to me.
My only advice is plan Lots of breaks to stop and use the bathroom. Also plan to not wait in the longest line, or get rider swap.
OP- Disney was my favorite vacation and I brought an under 2! I hope you find it just as fun and relaxing as we did!
I think this was more true when they let you bring 2 or 3 repeat riders. Now that it’s only 1, the repeat rider is generally only inconveniencing a single rider, which is only there to fill excess capacity anyway. In any case, rider swap is the only way to handle the situation where a kid can’t ride.
The only other option would be to make each adult ride alone, which they could achieve by waiting in the shorter single rider line. If they’re waiting the full standby line (or paying for ILL / LL), they should get to ride with someone.
I think I would be most understanding if you got in line with the kid, the kid got whiny/started misbehaving, and then you left for a while and came back later (rather than not even trying). I feel like at that point everyone around you would be like, “Yeah, go ahead and step out for a minute.”
I didn’t mean to get into a rider swap conversation, but I personally think yes. The adult rides alone. If your kid wants to ride twice your kid can wait in line just like my kid did. But that’s a different conversation. I was just trying to make the point that in this case my wait isn’t any longer. But it could be longer if someone uses the legal rider swap.
This is a great answer.
This is 100% true. I find it hard to be upset at polite people, no matter if what they did was completely rubbing me the wrong way. Things happen and a friendly disposition goes a long way.
Exactly 
I would much rather a cranky toddler next to me in line go get distracted somewhere else and pop back in later!
There were lots of times I was mildly hungry while walking by a mildly interesting snack item and didn’t stop because the line was too long and not moving. If I got in a short line and one person in front of me took forever to order because of discussions with family that hadn’t been there, I would be ticked. If they’d all been in the line to begin with and I’d have passed.
Indulge me because I think this is an interesting exercise in ethics.
Imagine there is a 60 minute line for Rock’n’Roller Coaster, with a 20-minute line for Single Rider.
So if three adults are ok riding separately, they could wait 20 minutes in the Single Rider line. (Imagine rider swap is like it used to work, where the non-riding adult waits on the side and hands off the non-riding child after the others are done riding). No one would argue this inconveniences anyone, since they’re just filling in unused capacity.
Instead, they want to ride together. So they wait an extra 40 minutes in the standby line (total 60 minutes). Should they not be rewarded for that extra 40 minutes of wait by allowing both waiting adults to ride with the third repeat adult? I think so. Yes, it is “adding” to someone else’s wait (specifcially, people who are in the Single Rider line) the same way that if someone else gets in line in before you do, it’s “adding” to your wait, but it is a totally legitimate and fair addition.
(Side note: while looking this up, I found out that @ehsanchez is in the parks today - she submitted an actual wait for RNRC. Do you have a trip report going?!? I hope you’re having fun!)