Andy and Jennifer Naselli recently reviewed theology books for younger children. Their reviews can be found here.

To their list I would add and highly recommend the following set of 4 books:

Cheryl Fawcett and Robert C. Newman (Schaumburg, IL: Regular Baptist Press, 2003)
Kids’ Questions about God and Jesus, Kids’ Questions about the Bible and Creation, Kids’ Questions about Sin and Salvation, and Kids’ Questions about Church and the Future

I’m using these books right now for our family devotions. Although they’re a little young for our kids (10 and 8), they are great for kids 4–8.