Of course the child is going for YOUR benefit! Your child asked for a friend to come along, you obviously think your child will be happier if she has a friend, you're including her in your family for the week. This is for your child's benefit -- not the friend's benefit.
Of course it's the parents' responsibility to pay for their own children on THEIR family vacations; however, I don't see that they have any responsibility to pay for a portion of YOUR family vacation.
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This is ridiculous. It is not as if she is asking for a baby sitter. Inviting someone to dinner is one thing, but a vacation is different. You aren't actually inviting her, but asking her parents if they would like to send her. Perhaps some rich people can say "hey would you like to send your daughter with us, on us?" But in this case it is more of "Hey, we're going WDW, if you want to send your daughter with us we wouldn't have a problem and DD would be happy. You would only have to pay airfare and tickets since we are taking care of the hotel"
Most parents would love the chance to send a child on an extra vacation. And most would also expect to pay. If your worried about it being less money the last time, then you could say "We would love your daughter to come with us again, but since we are flying it will be a bit more money. It will cost XXX $, let us know what you think.