We were/are always just honest with our kids. No, we don't think there's a god/goddess/whatever. When we/grandma/grandpa/whoever dies, they're just gone. Nobody knows where the universe/world comes from. As far as anyone knows it's several billion years old. Explain the big bang as best as we know it. Etc.

There's no 'right' way for so many things when it comes to kids. Ours know that many people believe in a god. We don't. That's ok.