more precisely, the system should recognize when a reservation includes an underage child, and require a seat selection adjacent to an adult on the same PNR. any attempt to seat a child, by either the passenger or the airline, separately should be prohibited. in the event of IROPS, equipment change, or something else that shuffles seats, the system should put the child+adult into either gate-reserved seats (e.g., bulkhead or last row), or move them up to economy plus or whatever.

i am not aware of any reason why the airlines can't do this. they just don't want to.