If I were you, I would seriously consider having a quiet chat with your mum about your concerns, but in a kind way. Explain to her that you and DH have had a long discussion about your expectations regarding the children, their behaviour, eating habits etc and that you plan to be somewhat more relaxed than at home and that you hope she can assist and support you in that.
Explain that you want to make sure EVERYONE is having a great time and that everyone needs to be respectful of each other.
Maybe hold a family meeting. We kind of did that, but our main family motto was:-
- Have fun
- If something you are doing is making someone not have fun, then stop
- Listen and use your manners
- Have fun
- Hands off
- Have fun
- At least one piece of fruit and one vegetable per day
- Speak nicely
- Have fun
We also had discussed in the lead up about what an expensive thing a holiday like this was and how hard mummy and daddy had worked to take the kids on such a grand adventure. How lucky we all were to be going, how most kids don't get to do this sort of thing, yadda yadda yadda.
We of course only have 2 kids and I'm sure the dynamics of 4 kids is a whole other kettle of fish. It sounds like you have put some great plans in place to keep the rowdier ones separated so you are already ahead of the game.
And as for DISNEY MAGIC it totally exists. You can almost feel it and I actually think nighttime is when it is at its most special. Watching Wishes Fireworks is one of the most amazing experiences ever. My DH doesn't even like fireworks and he said "wow, now that was pretty great".
We did have a couple of times where my son was a handful, my DH was a bit annoying at times too

but I just tried really hard not to dwell on it, to focus on my own experience as well because at the end of the day, all hell can be breaking loose around you but it's how you react to it and let it affect you that makes the difference. You will see the magic, without a doubt. I have no doubt that throughout the trip so will your children and DH and DM. But don't stress about it, just let it happen.
You lucky thing, I'm very jealous that I don't have a trip coming up.
