The Scriptures are fantastic for 'teaching and training in righteousness' (2 Tim 3:16) and for 'giving prudence to the simple, knowledge and discretion to the youth' (Prov 1:4). I highly recommend picking a passage (for instance, Jesus' resurrection recorded in Luke 23-24), reading it through twice, then when the opportunity arises reciting the story from memory (not verbatim just from memory). I do this with our kids and they ask all sorts of questions (as little ones do), some theological hard to answer, some silly, some completely off the wall. For passage ideas, I tend towards accounts of God's miracles (eg: the Flood), God's providence (eg: David and Goliath), and Jesus' parables (eg: the good Samaritan).
Of the kids books that we own, these are the most practical and most faithful and our kids enjoy them (4yr, 3yr, 1yr): The Ology by Marty Machowski is a systematic theology for kids and God Made All of Me by Justin and Lindsey Holcomb is simple, gentle book for preventing sexual abuse.