you could show them.
I've never been big on keeping things secret from kids.
My kid knows what guns are, and knows not to touch them. He doesn't own toys that look like guns.
I'm not saying I'm going to do it, but I'd think you could show them what goes in the safe, tell them it was stuff for emergencies, lock it, and tell them not to tell their friends or get into it on their own.
Like Rob said, if you can't trust your kids not to break into a safe you keep in their room, you can't trust them not to break into a safe you keep in yours - and you probably have bigger issues than that.
si vis pacem para bellum