It's tough isn't it mate. I'd suggest respond with just a "Sorry, I can't make it, regards C7". Remember, she chose this mate, not you. Why not make up a really cool Halloween activity you can do with the kids when they're with you again? Surprise them at changeover by getting dressed up in something Halloweeny and throw lollies around that they can catch!
Me: early 40's XW: nearly 50 T: 15 M: 5 BD: Jan 19 S:10 SS: 22 SD: 24