[Don Plumley]

Great question. I've been on both sides of this situation.

This has nothing to do with wealth and everything to do with common courtesy and respect: If we invite a friend along with us on vacation, we've invited them to be "part of the family" that week. I do appreciate when the parents initially and after offer to compensate for their kids share. I really appreciate when the kid says, "oh, please let me pay my way." But after their first offer I'll say to them, "Thank you so much, but the week is on us so please just enjoy yourself."

If the teenage kid is in a gift shop or something, then I expect they are paying for their own souvenirs. If we go anywhere with admission, airfare, food, etc., I don't expect out guest to touch their wallet.

I think you should contact the parents (in advance would have been better) and make the clear offer that you would like to pay your son's share of expenses. And after the trip is over, call them up and make the offer again. I expect it will be declined. Then send a nice thank you note along with a simple gift as an expression of thanks for their generosity. And you son should write the parents a simple note as well.

IMHO, sending money without discussion is a little insulting at worst or awkward at best.