Indeed, what the OP is proposing is not possible. Imagine that it is authorised and then the OP cancels the AA award booking, and you would then be left with one infant having been authorised a booking on his/her own (and at a pretty good rate too) to seat on some random passenger's lap (the 10% infant booking does not come with a seat)! I'm sure that would put quite an atmosphere in the cabin!

So as others suggested above, if you want you can do 1 adult+1 infant on Avios, 1 separate adult on AA or one joint PNR for all three of you, but not two adults on one side and the infant on his/her own.