I believe that you can start this habit early with your kids. Time with God can and should be part of our every day life with our children. When my kids were just toddlers/preschoolers, I found a great devotional. We even used it in pre-k and kindergarten.This is still my favorite book for little ones. We read ours MANY times (even broke the binding). It uses real life stories to help kids learn to obey and have a heart for God. It is not a Bible story book, but more situational. We used ours as part of our story time in the evenings. To help the kids stay engaged, you hunt for "Nightlight" a little glow worm on each page. You can order it from Amazon for about $10 new (HERE).
We have also tried the God and Me devotionals HERE (for girls - the boys version is Gotta Have God HERE).
These are also situational books for kids. They break them down into different age levels, 2-5 year olds (can't do with out parents help), 6-9 year olds (my older kids have even done these) and then 10-12 year olds. They also have a second book for each of these levels (like God and Me 2). My girls are old enough to do these on their own and we have done several of them.There is a little story, a verse or passage to read, questions to answer and then some kind of activity or craft. My kids have really enjoyed these. If you are looking for something that your kids can do on their own to start the habit of alone time with God each day, I would recommend these. Also about $10 each and you can find them on Amazon or in some Christian book stores. I also get these at homeschool conventions and have found them at yard sales or Goodwill.
My all time favorite is the Precepts for kids series (HERE or HERE). I have shared about them several times before (HERE). But it is worth mentioning again. This year we are studying the names of God (God, What's Your Name? - HERE) and we have all really enjoyed the study. These are a bit more advanced. Even though they say they are for kids 8-12, I really couldn't see my 8 year old doing these on her own. We do them together for group study time.
These books teach your child (and you too) to study God's word. How to really study scripture at such a young age. What a fantastic thing. We read the bible passage, mark the text, and answer the questions. We work through these slowly and don't ever do a "days lesson" in one day. We usually take several days to accomplish the lesson. I usually do the reading and they do the writing. I absolutely love these books. I can see older kids (older than 12) still benefiting from them.
Just wanted to share some of the tools we have used to hide God's word in our hearts. I have been recently challenged of how purposeful I need to be to do this with my kids.
You shall therefore impress these words of mine on your heart and on your soul; and you shall bind them as a sign on your hand, and they shall be as frontals on your forehead.
You shall teach them to your sons, talking of them when you sit in your house and when you walk along the road and when you lie down and when you rise up.
You shall write them on the doorposts of your house and on your gates.