I don't think anyone let him "play with it", from what I understand reading this the mother stepped out and the kid knew where it was kept, IMO stupidity on the part of the parents, even more stupid that it was kept loaded.
My son had his first .22 at around that age too, but he was never left alone with it, and it was kept unloaded and put away with the ammo in a different location he wasn't aware of.