I think about this question a lot as a family pastor and father of three young kids. My answer is pretty simple. Every night before bedtime, I try to gather my kids together and read from a quality Bible storybook or the Bible itself. I try to ask age-appropriate questions at the end, and then we pray together. That's it. Sometimes I'll get out my guitar and we'll sing a few songs. Other times I'll let my kids draw what they hear from the story and we'll talk about it. This doesn't take a long time, but I'm convinced that over time (by the grace of God) it is perhaps the most important thing I can do to disciple (or spiritually invest into) my kids. One other thing I'll add ... I'm slowly learning that I do my best parenting on my knees in prayer. Take a look at this post to learn a simple way to pray for your kids daily.