I have done this a number of times over the years. My son is now 21 but from the time he was 8 he generally flew award and I flew paid tickets.
You cannot link the PNRs (as others have noted) although make sure the award tickets are on the same (one) reservation (PNR).
I always called to make the original reservations: paid for me and award for child so there was no UM fee.
Always asked the reservation agent to note the PNR for the other reservation (cross reference). As others have noted did nothing in the system.
Then I would babysit the reservation. Only once was there an issue when they switched his SAN-LAX-SYD flight to SAN-SFO-SYD flight. Called and was told me it was not possible this had occurred BUT the notes were in the system. They switched me to the same flight for no fee.
Good luck travelling where the number of little ones outnumber you.