We invite my daughters friend on most of our family vacations because her family does not travel for fun. Usually its by car and I pay for everything for her except souvineers. This summer we are going to California via airplane for 2 weeks (she has never flown before). When I invited her on this trip, I presented it like this..."We are taking a trip this summer that would be a great experience for your daughter.
If you will buy her an airplane ticket and supply her with money for food since we will be eating out for 2 weeks in restaurants, she is welcome to join us".
They said yes, and went right on line and bought her an airplane ticket. I think as long as the parents communicate clearly then its ok to ask for air fare and food money. They asked me what to budget for food for her and I said $25. per day, and she can keep a log as to what she spends. Good budgeting experience for her. She will have a great experience and stay in some nice hotels and visit a state she has never been to for 2 weeks for probably under $500. Sounds like a great deal to me.

If it's a simple trip, with lodging and meals as the main expense, I usually spring for it. If there's activities like theme park tickets, ski lift tickets, water parks, etc., then I'd speak to the parents in advance and see if they could help out with those expenses. I think all haven't had an issue with it.

If I were on the receiving end of this offer, I'd feel a lot better knowing that my child would, in providing company for your child, be easing the burden on you, not increasing it.

Any other scenario, and I'd feel obliged to refuse your kind offer, given that for all kinds of reasons (and not solely financial - although that could be a factor) I might not be able to reciprocate your hospitality to my child.

Best of luck with your decision.

Thanks again for all of your replies. I appreciate all of them.

Pay for it all. Life's too short to quibble over a few bucks. However, just out of principle, don't pay for the tatoos all the kids sport these days or those ridiculous baggy shorts the boys wear down around their ankles. No belly button rings for the girls either.

Who knew tattoos and belly rings were considered a vacation expense?

GeorgeW, LOL!

Actually henna tattoos have become a vacation expense for many teenage girls! Hair braiding is another expense.

I second the recommendation to get written authorization (some places require a specific medical power of attorney) for medical treatment. This is often an overlooked consideration.

I work two blocks from George Washington University here in Washington. It seems like half the coeds on the campus sport tatoos on the small of their backs, right above their rear ends.

I suggest investing in laser tattoo removal technology. In about 10 years, you should be rich.

My son went to Orlando last year with a friend's family. I paid for the flight and gave the Mom $300 for admission into the parks. I also gave my son $100 in spending money. The mother offered to pay for his flight but I wasn't comfortable with that. I think with a road trip you should take care of lodging but I'm sure the parents will offer to chip in for theme parks, etc.

I have to third that a notarized letter of authorization to travel, as well as power of attorney for medical care, is very important. We do that even with trips with my husband's own daughter since he is not the custodial parent.

Statia and Sunshinesue are most wise.

Oh yes, the medical authorization is important. I also send my son with a passport, particularly on flights when I am not with him. He doesn't have a school id, too young for a drivers license and I just don't want there to be any travelling issues. By the way the medical authorization, insurance card and passport are given to the adult to hold!