When I had lap child I brought on a car seat. If there was an extra seat then I would put the lap child into the car seat, if there were no extra seats I put my 3 yr old into the car seat.
There was only 1 time where I had to put the 3 yr old into the car seat. Another time there was 1 extra seat on the whole flight and the stewardess has rachmanus on us and made sure that we got the extra seat.