DD15 is an only child and we take a friend whenever it works out with one of her friends schedule/family. I think it really depends on the family going and the personalities of your child and their friend. We are doing a third trip this summer with DD15's friend. She has gone to Aulani, DLR, now WDW and DCL with us. I'm excited for DD15. We've done a total of 10-12 trips with friends and not really had any major issues. I agree with KristinU that it is a lot easier if you have at least one pre-meeting. We've done this to make sure everything is in order. We understand that letting your child travel with another family is a big decision and when we ask we just accept whatever answer comes as graciously as we can. As to asking to pay for all or a portion of the trip, I don't think that it is inappropriate. We have never done so, but if a family wants to, we don't get in their way. Also, we always talk to DD15 first to ask if she would like to bring someone and if so, who. We ask her not to tell her friend and speak with the parents first, so the parents don't feel pressured in their decision. If I had three children, I don't know how well it would work, but I thought I'd just add my experience to the mix.