I believe many parents today think their children are too young to understand the gospel. Yet many children understand faith at an early age. Mnay dont realize God has given them on of the finest tools for teaching spiritual truth - the family. Kids can learn about Biblical truth through their relationships with their parents and their siblings.
Even the deepest truths if the Book such as Romans can be brought to life to a child. For example, kids learn of their mistakes within a family. They see their own tendency to be selfish, to disobey and to sin within a family. You can explain a verse such as Romans 3:23 to them by using their selfishness as an example pf what it means to "fall short of the glory of God".
The family is a divine incubator for teaching spiritual truths. We need to use it when teaching the fundamentals of faith to our kids.