It's always great when children are inquisitive and like to explore!
LR can only be locked during playback, when banging on the keyboard won't interrupt play (by selecting "Auto-Forward" and "ESC key only" in the playback settings).
But seems like you're looking for more than that, which unfortunately is not available. If you're afraid of your son messing up stuff when you're not around, then easiest thing would be to password-protect your screensaver.