Under guidance of our dad and grampa, we learned to shoot an pump air rifle before we were big enough to pump it or strong enough to aim it -- rested it on a ladder rung. As far as reloading, when they are curious, have them watch, at a safe distance, with protection.
Like said above, keep powder and primers locked and the key inaccessible.
But further, all ammo needs to be locked up, too. A curious kid will steal a cartridge, pull a bullet with pliers and a vice.