Possibly introducing them to some of the content in A Human's Guide to Words, such as dissolving the question, would lead them to theological noncognitivism. The nice thing about that as opposed to direct atheism is it's more "insidious" because instead of saying, "I don't believe" the kids would end up making more subtle points, like, "What do you even mean by omnipotent?" This somehow seems a lot less alarming to people, so it might bother the parents much less, or even seem like "innocent" questioning.