I can't say from my own experience; still married and it's 1st marriage for both dh and myself so I won't be offended if you tell me I have no clue what I'm talking about but...
I'm guessing there will be many Firsts that you and your ex will both be missing out on and that's just the way it is when people get divorced. For right now it's their 1st Disney trip. Maybe your ex will take them on a
Disney cruise and you will miss that First. or maybe their first bike ride with no training wheels or their first time boogie boarding at the beach, first ATV ride, first ski trip... You get the jist of it. You both can't always be together for every first experience in life.
Again, JMHO but I think this should be you and your boys together. This is beautiful quality time without the stress of everyday life getting in the way. If your ex wants to plan a separate trip, let him. This was
your idea and your vacation.
Heck, it was hard for us last Nov. when my parents, brother, and SIL came with us. Naturally people want to do different things and it becomes a committee decision for every little thing: where should we eat, what attraction next, what time should we leave, stay to see Wishes? etc. I think it just give more opportunities for you and your ex to disagree and give his new wife control.

Ugh, I'd rather be at work.
It's one thing to "take the high road" and compromise with some things for the sake of the children. If you can share the cost of a Chuckie Cheese birthday party, that's great but that's about as close I'd ever let things get.
Enjoy your vacation with your children!!