X is a manipulative ass. Do not engage with him on that level. Ignore his comment about "putting the kids first." Business only. This means not taking his bait.
Now, unless you want your kids to spend July 4 with X, his GF, and her kids, I'd suggest you stick to your plans. And no, this does NOT mean that you should find out whether they will be there or not. Simply assume that they very likely will be there. X has demonstrated that he will expose your kids to them and that he will not be honest or direct with you about it. You don't need to learn that more than once. Take it as a given.
Moreover, you HAVE already made plans. And, you don't need to explain anything to him. Nor should you change your plans for his weekend trip. Whatever you do, do NOT get into the game of justifying your plans or why you can't move them and so on with him.
"X, sorry, that doesn't work with the plans I've made for me and the kids. So, we'll need to stick to the schedule for both the weekend and for July 4. BTW, this also includes Friday evening as I'm booked. So you'll need to make some alternative childcare arrangements to cover your time to make sure the kids get dinner, etc... With more notice, I hope we can both maintain some flexibility, but in general it is probably best to stick to the schedule."
