I got to fire some shot shells with an M79 40mm grenade launcher once, rounds had one layer of 1/4" lead *****. I think that was the hardest kicker I ever shoulder fired.
My kids all learned at an early age. Boys had their 1st BB guns at 7-8 and I had a range set up in the basement. They would go down there and shoot all the time. ...Got some holes in the ceiling tile....and a few in the celler door, but generally they were pretty safety conscious and learned pretty quick what not to do!!!