Well when my children were young I was still, nominally, a believer. If I had raised them without theism from the get-go then I probably wouldn't talk about it much. It would be more relevant to explain that most people have some form of magical beliefs in invisible beings and realms (both conventionally religious and just general woo like hermeticism) -- and why. Judging from my grandchildren, who were raised in an unbelieving household, this would be the way to go. The conversations they've initiated with me on the topic are more along the lines that they are baffled why so many people believe such nonsense. I try to give them my insider's knowledge of the mindset, and explain the epistemological and theological stew that they live within (in terms that are age-appropriate).

I also teach my grandchildren to be respectful and accepting of others, to treat them kindly even if you can't respect their beliefs.

I never did, but when they asked question I told them the truth and asked them what they thought.
My son declared himself atheist at 4 years old after he asked me what a person who does not believe in god is called, my daughter has been a Christian (sort of) all her life but still asks questions and listens to the answers.

How old are they?