I was going to suggest the same as Ocean. Don't tell anyone what you have planned and it'll probably drive her nuts. And if the kids ask, just tell them you haven't decided yet, that's all.
You'll let them know when you do. Because she'll probably hound them till she gets information. My son deals with this too. It's a pain in the butt. What he buys, she buys. What he does she does.
Last year, for their sons birthday she TOLD him, didn't ask him, TOLD him that she was making the plans and that he would be paying for half. His family was invited if they wanted to come but that her entire family would be there and all her friends. Much more than his side. She was going to have quite the spread and she had no money. NONE...So she was expecting him not to foot the entire bill.
He told her NO WAY. She got so mad.
He told her straight up, why should I come to a party where I'm not wanted first of all, and where I'm going to be paying over 500 dollars and where I"m going to be ignored and where I probably won't get any of the food? What the heck? He told her no forget it.
How it works with these two is he gets every weekend visitation. She is a stickler with her days and will not let him have ANY extra time of her time at all what so ever!!! none. She used to. But that has stopped. They had an argument because he wanted to do 50/50 and she flipped out.
So now she won't allow him any extra time. Now he's got the weekends, and this summer when the birthday rolls around he's not going to allow her time to celebrate on the weekend, she wants to do it, she can do it in the evening, or she can do it during the week. That's her time. She wanted to be a hard nose, so this is how it goes. Right or wrong. This is what you get.