On Southwest?
I agree with a rule for the sale of assigned seats. Using differential fares to essentially extort more money from parents with young children is contemptible.
But without assigned seating, and explicitly no guarantee of any particular seat nor any price difference between any seats, FA's are empowered to shift passengers for exactly this purpose. And they are in no way shy about exercising this power to ensure young children aren't seated away from their parents. I flew WN at least twice a week for five years and saw it - was myself asked as a single passenger - innumerable times. Over 500 flights, never ever saw a child separated from a parent.