I've had the stewardess notice my kid in an adult only seat (turns out only one side of that plane has airmasks that fit kids or something like that) and move us, then tell whoever we displaced to take our old seats.

If I was a flight attendant, I’d use this reasoning regularly even if it wasn’t true. Hard to disprove on the spot. And these kinds of things can even vary from order to order of the same aircraft between (and within) airlines.

The idea is to get noticed early, if you have a seat assignment issue, or get a row if it's a free-for-all seating airline.

Again, it's entirely different if it's a toddler/small child vs a teenager; teenagers are probably happy to be away from the parents for awhile.