A gun safe is to keep the burglar safe.
Sorry, if the grandkids can’t learn to keep their hands off dangerous things that don’t belong to them, then they don’t need to be at anyone’s house. I’m betting the vast majority of FReepers were raised in homes with guns and we’ve all lived to tell about it.
I second that. Training the grandkids is superior to a safe. I grew up around loaded guns (and sharp knives and explosives and antique heavy machinery without “safety” devices). It never crossed my mind to play with a gun or a knife or the hydraulic press without first asking permission. I had seen what these machines could do and I was a bit terrified of all of them.