I used to keep mine locked up in a cabinet, not the safe. The powder has always gone in a cabinet. The ammo won't fit in the cabinet anymore, so it's on shelves.
My kids are older now anyway. They know better than to do something stupid like chock a round up in a vise and hit the primer with a nail. Which brings me to this:
I don't see why ammo would need to be locked up. It is not going to do anything by itself. We never had an issue with our kids, or when I was a kid, by having ammo stacked on shelves or in unlocked cabinets.
Nothing happened in our house either, because Dad told us what happens when you chock a shotgun shell up in a vise in the basement and hit the primer with a hammer and nail- he learned from experience when he was 7 years old.
If you want to know happens, apparently, it's a ringing in your ears, a torn pant leg, and a whoopin'. Didn't sound like fun to me, so I never tried it for myself.