My kids tend to ask big questions, and they start young. Back when my oldest daughter began asking about G-d, somebody recommended Where Are You, Hashem? by Yaffa Ganz. My daughter loved it, and so did my other children. It’s a great introduction to the concept that G-d is everywhere, for ages 2-8.
Another, more recent, book that introduces children to the same concept, is Hashem is Truly Everywhere by Chani Altein, illustrated by Marc Lumer. My husband got it for my son when he was two years old, and he really enjoyed it. The illustrations are adorable.
