I believe these two things are true (please correct me if I am wrong):
-kids under 3 years old do not need a FP+
-fast passes are generally harder to get the larger your party
However, it looks like Disney still allows you to select guests under 3 when making FP selections (I am not at my window yet so again let me know if I’m wrong).
Why? Is there any advantage to including someone under 3 in a FP reservation? What am I missing?
Both true.
You can only get a FP with a valid ticket, and under 3s don’t need one either to get into the park or to accompany someone with a FP on the ride.
And with larger groups it can be helpful to overlap FP - you might not be able to get 10 FOP at 2pm but you might be able to get 3 at 2pm, 3 at 2.10pm and 4 at 2.20pm. Then everyone could ride together any time from 2.15 (using 5 mins early grace period on latest FP) to 3.15pm (using 15 mins late grace period on earliest FP). You’d have an hour in this example but all you really need is 5 minutes where all FP are valid even if they’re at different times.
I got worried after FP day because DS1 wasn’t on any of them. Seemed weird at first but then made sense. When I made the selections, I had him checked at first but the next page said I needed to correct this problem (guest does not have a ticket). So of our group of ten, our FP choices were for groups of nine. I hope that makes sense 
You can select the guest under 3 but it’ll show an error message on the next page. Just make the fast pass for whoever will be riding with the kid.
Thanks all! That makes sense.
I was getting that error but I get the same for just myself since I am more than 60 days out so I couldn’t tell.
Thanks for the tip as well on overlapping FP for the subgroups. I’ll be coordinating a group of 10 (really 8 since two are under 3). Is there an optimal number you want to divide into? (e.g. four groups of two versus two groups of four) Or is it just trial and error, brute force and luck? We’ll be there between Christmas and New Years so I’m aware it will be a battle.
It just depends what’s available - you just need to be flexible. But at that time of year for that many people it won’t be easy.