Did you ever say what is exactly going on with these sports in the summer? Is it in season games or practices?
What would have been the best way? I told him last year about it and he seems all good about it, even told his daughter if there was a camp she wanted to do here that he would consider it. He even told her to put together a plan so that he can still see her and she put alot of thought into it, last Christmas when she was with him and he kept saying no, no no no no.... He just keeps stringy them along.
Honestly, do you really need an answer to this? OK, here goes ....
STAY OUT OF IT! You are way too involved in what goes on with your kids and Dad. Just stay out of it. It is what it is. You do not give Dad the respect he should have as a parent. Does he breathe down your neck at all your parenting choices? Even if he does, you need to back off. That's it. Period. If your children have feelings with regard to their Dad's parenting choices, just like their feelings about your parenting choices, they go into the buckets of "oh well, too bad", "well, we'll see what we can do about it", or "I made a mistake". You don't get to choose which of those buckets their disagreements result in with Dad just like he doesn't with how your kids feel about your parenting choices. They will work it out in the long run if you stay out of it -- well, that is if the kids stop TELLING Dad like you do. Asking someone a question that has only one "right" answer is not giving them a decision, I'm sure you know that. You keep triangulating the relationship and never with a good result. If Dad lived closer -- which he can't because you moved his kids away from him -- he could give you a dose of your own medicine by planning all kinds of activities on "your time" that the kids would like to do and have the kids want to opt to go with him over you time after time, you know, their choice and all.
It doesn't matter how valuable you think the activities are, it's not your decision and you have to get that through your head. That is what happens when you are not married to the other parent where you could exert leverage when you disagree on parenting choices. You have no leverage now, you have a court order.
Your kids will not shrivel up and whither away if you just stay out of it. If you really think you are doing them some big favor by causing discord with their dad, you are really full of yourself. Not saying this to be drama filled, I'm looking at this objectively.
And, if you pull a stunt where you pretend like dad's not entitled to their entire summer time off school just because dates aren't specified, you better hope Dad doesn't have an attorney because you will be in a whole lot of trouble and may find out what it's like to spend time in jail for contempt or have custody changed. And, beyond that, it's just wrong to be so overinvolved in Dad's parenting decisions so just back off and let Dad be Dad to these kids (remember, you chose him as their Dad so quit trying to undermine him now).