It was when I saw a mother and child (on Ryanair) being moved from the exit row and swapping places with people in Row 1 that I realised that these premium seats often do *not* sell and get allocated to the late check-ins. Because obvs with a child they would not have been able to *buy* an exit row seat.
Have seen parents book their child as a adult so they can get the emergency exit
As for paying for a seat have never paid for one, My wife is a WCHC and needs help in flight just phone up the call center and explain to them and get allocated seats, on a LoCo often row 2-4 on a wide body often 2 seats on their own were the config allows have also been give a seat with room for a infant basket once.