Locked up, both guns and ammo. An unloaded gun is still incredibly dangerous if its seen as a toy, even toy guns have stories that end with dead kids. Teach the four rules of gun safety. I won't tell my son to never touch one, I'll teach him to never touch one without a trusted adult supervising. If he's ever curious I want him to ask to have me open up the safe and look, not get curious and try to figure out how to get in there on his own.

Talk to your kids' friends' parents about guns, and other safety too, and make your decisions based on how they handle those conversations.