parents would at least offer something to the cause...after all the parents would be going out of town regardless if the child stayed there or not and some cash was going to be put out there anyways either for some kind of sitter or to pay the child's way on the trip. would i expect the other parent to pay or offer to pay?....probably not because that's just how i am. it would just be the "nice" thing to do.
i would allow it with no hesitation and without expecting anything in return. why not help someone when they need it? plus, they may help you when you are in need 