A small child won't understand everything about God or the Bible -- but they can understand something, and we need to do all we can to teach them about God and His love.
After all, how old does a child have to be before they understand a parent's love? Do we have to wait until they are older to let them know we love them? No, of course not. They sense our love almost from the moment they are born -- and as the years go by they'll understand it even more.
This is similar to God and His love for them. Almost as soon as a small child begins to talk, they can begin to understand that God exists (even if they can't see Him), and that He loves them and takes care of them. (After all, they already know what love is because of the way you've shown it to them.) They also can begin to understand Jesus and what He did for them, and with child-like faith they can learn to pray and trust Him.
Jesus said, "Let the little children come to me, and do not hinder them, for the kingdom of God belongs to such as these" (Mark 10:14). Don't wait, but begin now by teaching some simple prayers to your children, reading to them regularly from a children's Bible storybook suitable for their age, and most of all, being an example to them of Christ's love and truth in your daily lives.