There are a few books that we read our kids almost every day. They are edifying for all of us and I highly recommend them to any Christian family with one or more children, ages 5 and above.
1. Big Truths for Young Hearts, by Bruce Ware--this is a systematic theology book written in concise, simple language so that children and adults alike can understand "big truths" about God. I wholeheartedly agree with A.W. Tozer who said, "What comes into our minds when we think about God is the most important thing about us." In my opinion it's never too early to teach kids the truth about God. They think about Him and ask questions about Him, themselves, reality, heaven and more--so why not lay a solid groundwork from the very beginning. Additionally, you as a mom or dad will learn a ton too. Other than the Bible itself it's my highest recommendation for parents of school age kids.
2. The Jesus Storybook Bible: Every Story Whispers His Name, by Sally Lloyd-Jones--awesome kids' bible. Let me be frank: there are a lot of stupid kids' bibles out there. Don't get one of those. Be educated and get your kid a bible that isn't going to water down the character of God. Most kids' bibles make the sovereign king of the universe look silly. This kids' bible is beautifully illustrated, well-written, theologically sound, and captures the picture of Christ throughout both the old and new testaments. Mark often reads it to the kids before bed.
3. Operation World: The Definitive Guide Prayer Guide to Every Nation, by Jason Mandryk--this book will broaden your worldview and increase your concern for people around the globe. After a quick theology lesson from #1 above, the kids and I always read the nation listed for the day in this large book. It has opened our eyes to countries and people groups we never knew about. Each day provides a brief lesson in social studies and church history. Together we pray for the countless people around the world who are poor either spiritually or physically. It's truly eye opening.
If you don't want to get the book, Operation World has a great web-site including Prayer Videos for many countries. These are 3-minute videos highlighting the nation's prayer needs, as well as a national praying for their country.
4. And don't forget the Bible.