We had a similar experience whilst cruising along the Staffs & Worcs canal with our friends.
Whenever we go cruising together, we always help single handers along the locks knowing how difficult it can be for them. On this occasion, while attending to the other boater, we'd opened the bottom paddles of the lock my wife was waiting to enter, in readiness for our return so that the lock would be set. Some of the crew of the boat behind ours (a man and his two teenage daughters), proceeded up to the lock and opened the gates to allow my wife to enter.
Once safely in the lock, my wife thanked the chap but asked if he would discourage his girls from raising the paddles, as our 40 footer had a tendency to lurch back and forward in the chamber if the paddles were raised too quickly. Obviously in a hurry to progress, he ignored my wife's plea and allowed his offspring to rack up the paddles with great enthusiasm. As she'd previously experienced, the boat shot forward then back with the onsurge of water.
Returning to the lock, my wife's expression was enough to explain what had transpired. Because we were going to be leading the way infront of this chap along the series of locks, I reminded him about the dangers of inconsiderate lock wheeling and again asked if the girls could refrain from doing it. The chap, who was probably in his forties, took the huff and bellowed that he and his family had been boating for years and they knew exactly what they were doing! Just then, his elderly father arrived from their boat enquiring as to what had unfolded. He took his arrogant son to one side and had a quiet word in his ear.
From that point onwards, the girls would come along side the locks, await the signal from my wife then slowly raise the paddles. Their father on the other hand would skulk along with them, without uttering another word of his reputed wisdom.
So yes, I agree with your sentiment about children being supervised at locks, even if they are forty years old!