I really don't like short barrels for kids. The increased muzzle blast isn't the reason. If you make donning ear pro. part of the shot sequence they'll use it out of habit.
The reason is short barreled guns are more likely to be pointed at the shooter and bystanders. This is the same reason I don't let them shoot handguns. And the same reason I won't let a new adult shooter load more than one round at a time in a handgun until they demonstrate control of the weapon. Even experienced shooters have accidentally shot themselves.
My kids can't shoot any of their guns offhand. All shooting is done from a rested position. Part of our practice is moving the gun from rest position to rest position between different targets, putting careful one shot hits on target. These kids can hit at least as well and as quickly as the average adult deer hunter.