Well after 52,000 responses, you probably aren't even reading replies anymore. And, you've already read responses that cover all bases. I just wanted to say that (in a very small way) I can relate. I have a very similar setup to yours, however, the mom with kids in this case is my DGF who moved to Illinois from Texas with her 4 girls and they all live with me now. She has hardly any money, but announced to me a few months back that we are going to Disneyworld this summer (which I love) and we are surprising the girls. Some time passed and it occurred to me to tell her to make sure she does NOT tell her vindictive ex what we are doing. He seems to be as big a jerk as your ex. and may attempt to do this while the girls are visiting him over a spring, summer or christmas break in TX. So she didn't tell him, but did write something to a Texas friend on facebook and I told her to delete it as this person may mention it to the girls or the her ex. She deleted it and we are being SO careful now. They leave in a couple of weeks for a 3 week stay in Texas, come home and then off to WDW we go. I'm still slightly worried that he can mess this up for us, but I'm he hasn't given any indication. As for you, just go on the trip....your kids are SO torn up right now being there with him, that maybe even he will see what an awful thing he did (not to you) but to them as they were a big part of your planning.
Good luck
J