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. I have way more ammo than would fit in even a large safe or two. I have one HD shelving unit loaded with ammo cans of ammo and components and more ammo boxes on another shelving unit and a couple of cabinets. I keep some HD ammo with the guns but all the range and reserve ammo, and components are on the open shelves.
"The ultimate authority ... resides in the people alone. ... The advantage of being armed, which the Americans possess over the people of almost every other nation ... forms a barrier against the enterprises of ambition."
- James Madison