Surely if they are unaware of the fact, they will presume any "friendly reminder" is intended for other parents? So it is pointless.

People who are courteous enough to heed such warnings generally don't need them.

Where a cabin's peace is destroyed by children it tends to be the parents' fault in my experience.

I've had more flights ruined by inconsiderate adults than children. Perhaps we should remind all posters to behave on flights.