It's frustrating that AS can't handle this case well for certain things like selecting Premium class seating with multiple elites on one PNR, but alas. As a family of 4, we just book two PNRs of 1 adult + 1 child, and it generally solves the problem of both free Premium class seating and upgrade possibility. For you, the main deciding factor on which adult to hitch the kid to is would you rather have 1 or 2 of you up front (assuming upgrades are an option)?

You can also book all 3 tickets at once and then call in to have an agent split one adult off into a separate PNR. If you do book separately, you can go into one of the PNRs and choose the Link option and enter the other confirmation number to link them together. It doesn't do a whole lot, but it does let agents know you're travelling together.