I apologise in advance for the length.
Tldr; go in armed with a plan that everyone is committed to.
I've always been pretty firm with my kids, and despite this my youngest still had what looked like explosive tantrums (ASD meltdowns) right up until he was 9. He turned 10 last week.
First, before anyone says anything, I KNOW a meltdown and tantrum are different, however after seeing my brothers with ASD as adults after having their condition used as an excuse, my DH and I decided from the beginning that yes, the explosions were likely to happen, but that they weren't going to be accepted without consequence.
We have rules at home. We make sure we tell the kids before we leave that the rules don't change on holiday. When they were littler like yours are, it was more constant reminders about what the rules are-both before and during the trip.
Consequences are given when the rules are broken. It might be missing a treat, not getting a toy, or for big ones like tantrums, leaving the park. If you say you will do something, follow through. It needs to be appropriate for each child and should sting a little. It's a deterrent after all! For my youngest, taking away his books is the most severe consequence we can give. It wouldn't bother my older boy and so wouldn't be a good consequence. Always follow up after to talk about why and a reminder that we do this because we love you.
Yes, we do remind or give warnings (only 1 each!) , and make sure they know what the consequences will be prior to giving it, if possible.
Adult actions need to be considered too. Sometimes our poor choices cause the kids to not cope. That doesn't make a tantrum OK in our family, but the consequence is measured against all the information.
Reward the good! Our boys don't get many extra treats, toys or sweets at home. If they're behaving and getting along well on our trip I'm happy to get them an extra ice cream or a small toy or something- and tell them why they're getting it!
At the end of the day, I'd rather spoil them a little for being awesome superstars and behaving themselves, than have them act out and not have a consequence. I am far more concerned with raising kind and decent human beings than anything else in life.
This is what has worked for us, and I find the boys are actually best behaved when we holiday. We spent two weeks crammed (half buried under gear in the back seat) in my lancer on a camping roadtrip, and they didn't argue once.
But it only works for us because it suits our family dynamic, everyone knows what to expect, what is OK and what is not, and everyone is committed to following it. Every faculty is unique and you will need to find what works for your family.
The big key for us is to have everyone on board. It doesn't matter what the plan is if someone isn't committed.