Two things:
They have been known to jam, and in an emergency you might not be able to get it off. I know of a case where a federal law enforcement agent had to go to a gunshop to have one removed. Most teens (and gunshops) can pry the lock off with a screwdriver.
Many one-size-fits-all locks come with instructions saying they shouldn't be used on a loaded gun. The lock can press on the trigger and fire the gun if it is loaded and cocked.
A better alternative where children are concerned is to keep the gun in a locked case, box or safe. Or you can snap a small padlock behind the trigger of a double-action revolver, or some pistols, and the trigger can't be pulled back far enough so the handgun can be fired.