Return tickets.

I guess my only option is to call Qantas and see if they will change the award ticket to the infants name.

I just wondered though what happens when a larger person wants to buy two seats on a plane. I assumed that was possible?

Similalry if you wanted to buy the seat next to you because you preferred not to have someone not sit next to you. I can't imagine an airline would be averse to accepting your money?