Sometimes a quiet word to the parents on what could happen in an explosive decompression is enough to get them to keep their kids seated. Not too many details, mind, but just enough to imply that little Kevin might go for a free flight of his own would do the job
Joking, of course.
Sort of.
Once had a couple of kids running amok on a flight, up and down the aisles, mum of course was too busy reading a magazine and drinking wine to care. Asked her several times to keep the kids out of the galley, with explanations of what might happen. Couldn't care less, obviously, as this behaviour continued for some time, with us having to interrupt the service to carry the kids back to their seats on a regular basis.
A little while later, we were preparing for the service and one of the crew was trying to get a stuck cart out of the stowage. She put her full weight into and pulled the cart, finally getting it free right at the moment that little darling ran back into the galley, whamming him with it and bowling him over. Kid lay on the floor for a second or two stunned, then proceeded to scream its lungs out, she knelt down and said simply, "That's what happens when we play where we are not supposed to" marched kid back to mum and we didn't see him for the rest of the flight. Of course mum tried to kick up a fuss and say she would complain but the whole thing was witnessed by an FF in the bulkhead row who turned around and told her he was going to write to the airline outlining the whole incident and she had better start to parent her kids or else! Love our FFs
Moral of the story, sometimes sitting kids with their parents doesn't work either....