We have done it with a young adult, but he is disabled. I don't think there is an age limit.
If the family is waiting in the line and then wants to swap because there is someone who "can't ride and can't be left alone", there should be no problem. It is no different than just letting some families go ahead of part of your group until the other half of your group has returned to take the child with them. In theory it should take longer.
As far as leaving them to wait, for some kids this may be 12 but some it may not be till much later. Especially to a child new to the park or maybe not used to being in busy places. I have an extremely mature 11 year old who I probably could let wait alone, but I don't think I would in such a busy place.