Alaska Airlines is brilliant in handling this situation: you can book the adult ticket first, then when you book the child it recognizes that there is no adult on the booking, asks you for the adult PNR, and lets you book the child on their own ticket. Wish AA had it working that way. If you can buy the ticket as a codeshare through AS, maybe try that (although I think this may only work on AS metal; I know it didn't work for an AS award ticket with an overseas partner airline). I do this routinely for companion fares and/or adults flying paid while kids are on awards, for example.