I've taught my grandkids to not touch any ammunition, firearms, knives (other than eating utensils), tools, etc without me or my wife being present. If they wish to see whatever has their attention closer, they merely have to ask and one of us will show it to them and help them to handle it safely or show them how it works. This seems to take a lot of the mystique out of potentially hazardous objects as well as teaching them manners, educating them on proper handling of an object, and how it is used. The item then becomes a non- interest for all intents and purposes as the kid's curiosity is sated.
If there was some unusual situation that this would not work then I probably would put a lock on the room door and lock it up and keep the key with me. This will work if the kid(s) are always under supervision and you always remember to lock the door after leaving but such things seem to backfire. Denying entry tends to pique a kid's interest and they will make every attempt to gain entry. They can be very resourceful and unrelenting in their attempts. Locking things up never worked on me as I always got entry to places long before I was given permission.
If one can't lock the door, then a locking file cabinet would work well. One can usually find them pretty cheaply at a second hand store, ad, or even word of mouth. One does not need anything special to keep ammo in.
