Why not just disable Family Share instead?
The issue is without family sharing, you can't share games. But with it enabled if you both own a certain game, it gets confused and only 1 of you can play it at a time.
But if I buy game Y, and play it, steam will then prevent you from playing my game X at the same time.
What's worse, even with hindsight I'm not sure how a parent should organize 2-3 accounts to allow all what "family sharing" implies. And consider that kid now plays some Mario game but in a year or two would play Wolfenstein, so it's not easy to separate by "my" and "their" games.