Don't store ammo in your safe, or a loaded weapon, either. In a fire, the boxed rounds will throw shrapnel all around the safe, and the loaded rounds will fire out the muzzle of the weapon, and either ricochet inside the safe or punch a hole in it and anything in the way on the other side.
As far as the kids go, they may be strong enough to swing the door open enough to get their arm or leg inside, then lose control of the heavy door. Keep it locked when adult is not present in the room to watch 'em.
In the event of a fire, you want to throw your Sentry paper box out the window and follow after it.