To me, you do NOT owe her anything in terms of being a babysitter while she goes to sow her oats. IF, and that's a BIG IF, you two somehow had a previous arrangement, agreed to before this trip came up, where you were to look after the kids for this week, then fine, but otherwise, as you said, welcome to the wonderful world of being a single parent. You can't just take trips and expect your ex-partner to pick up the slack. You actually have to plan for these things and then IF your ex is open to it, MAYBE they will accommodate your wishes...maybe not.
I am not saying to try to spite her but like I said, why do you owe her anything more than accepting that she's making decisions for herself? That's already MUCH more than a lot of people would be willing to do. If she wants to see your lack of "help" as trying to prevent her from going, well, I don't know how much that can be helped. If she really wants to go then she has to act like a big girl and arrange for care for her kids the times you can't do it because of work or whatever.
I know this is a tough spot. It's NOT that cut and dried because you are torn between acting "as if" and enabling her. It's a spot we are in a lot it seems. It SEEMS to me that there is a line here but I really think this is one of those times where only you truly know enough to make this decision.
