airfare and the park ticket (and of course if the girl wants souveniers, extras, etc.)
I would not ask for a portion of the hotel, food, etc.
I agree with this.
While I generally don't think it's a good idea to invite someone somewhere if your not going to pay for them, I think there are times when, if discused first, it would be fine.
It would be a shame for the kids to miss out just because you can't pay teh whole thing, when the other parent might be more then happy to contribute so the kids can go on a trip together!
I also agree with being up front, it's always the best plan.
Not really related but...
I had a friend invite me on a ski vacation with the family to their time share when I was about 14.
I said no because I knew my mom couldn't afford it at the time, but my friends mother said they would cover everything but the ski passes. Since they were driving and had a timeshare one more person didn't make much of a difference.
I ended up going, but found out after that my friends mom had come to my mom about the trip just days before leaving and basically said that if she didn't put some money towards the trip I wouldn't be able to go. My mom ended up giving her about $400 to cover food and gas for me, and agree to go to a condo presentation where my friends family have their timeshare (that way they get a discount).
That's on top of the $350 she gave me for food, ski passes and emergencies. (this was quite a few years ago now)
Needless to say she wasn't impressed when she found out I had bought all my own food.