I've also done this successfully in the past with award tickets, wherein each of my kids was on their own pnr, with my wife and I also on the flight, but on separate PNRs. They apparently can "link" the PNRs in their system somehow, so that someone looking at the child's ticket can see that there is an accompanying adult on the flight. Of course the linking doesn't help with seat assignments, so if there is an equipment change or some such, you may find the child at the other end of the cabin from the adult.