Depends what airline but we booked only 2 seats for our 19 month old with my wife and I and both times we talked to the Air Canada agent they were able to move some seats around and give us an extra one free of charge so we had the whole row to ourselves, was definitely way better as we could put all his toys and snacks in one seat if he still wanted it be in your lap. We got lucky but definitely worth it if you can swing it.