Discipline. For you, not the kids.
You need to discipline yourself to do a fwe things religiously. I have a young son with an inquisitive mind and have adopted these and it seems to work.
Everything but the HD gun is unloaded at all times when the kids are around. This will change as they grow up enough to go hunting with my wife and I. Ammo is in a locked box my three year old has never seen open. We've tested the "go get a gun" and "go get a bullet" requests. He'll come back with a handgun, but cannot find any ammo. Talk about incentive to keep them locked and ammo seperate!
CCW gun is unloaded unless it is on my person. When I go to bed or remove it for some reason, the chamber is cleared.
HD gun(s) is/are loaded only when the CCW isn't. When I go to bed, CCW comes off, HD gets loaded. When I get up, CCW goes on, HD gets unloaded. There is no loaded firearm outside my control this way. When I'm at work, the shotgun in the bedroom isn't loaded. Wife has a CCW, so they're not defenseless.