The good news is that exit rows are desirable, and children and their keepers are prohibited (in the US, anyway) from sitting in them. So they can pre-board all they want, and not take away any of the good seats.
(Of course, people are idiots. The one time I flew Southwest, I was on standby and boarded last -- the only remaining seat was an exit row. I was amazed.)
I recently flew Southwest and sat in an exit row. The flight attendant came by to give her spiel, and the 100 pound young woman sitting next to me expressed doubt that she could lift 50 pounds. I think it took about 5 minutes of pleading(!) with the rest of the completely full plane to find a volunteer to change seats with her.
shush... please don't reveal this well guarded secret... I love always coming late in the plane and still getting the exit row.
But if you start teaching other people that those are nice sits....
(Of course, people are idiots. The one time I flew Southwest, I was on standby and boarded last -- the only remaining seat was an exit row. I was amazed.)