My daughter is starting school in the US (we stay in Thailand) and my wife and I would like to accompany her back. There is a much better deal if we buy round trip tickets, but flying back would only be my wife and myself (contenting through China) . What will happen if my daughter doesn't use her ticket to return with us?
You'll simply forfeit the return ticket for your daughter. It might be smart to try to book her ticket with a far-away return date if it doesn't cost extra--maybe she'll come to Thailand a year from now?
If you're wondering if the airline will prevent you from boarding without your daughter present: no, they shouldn't have any reason to do that.