A child's mind is not an adult's mind. Children cannot understand abstract concepts as adults do because their minds are still developing. So, how do we teach a child about God, Jesus and the Bible?
Basically, a child sees his parents and grandparents as very powerful beings. These people provide food, shelter, education, toys, and love for him. How a child relates to and receives care from his parents carries over into adulthood and the young adult will view God the way he viewed his parents. So, the first thing we, parents, can do is provide great love, nurturing, care, and education for our kids. Our relationship with our child will provide the foundation of the child's relationship with God.
When a child trusts his parents, the child will naturally copy the parent's attitudes toward God. If the parent or grandparent is constantly talking about concrete ways God is blessing him, the child will hear and learn. So, for example, a grandfather may say, "Look at the beautiful blue sky God has made for us!" The little one sees the grandparent's happiness and associates gladness with God's creation. These daily comments are internalized in the child's heart and become part of his being. There is no need to constantly question to see if the child has finally got it right. This type of heart development occurs over years. It responds to love.
Of course, the young one will hear conflicting ideas from other adults. But just as we are all drawn to a person who tenderly, gently understands us, so the child will naturally long for the blessed touch of the loving parent. As he matures, he will weigh how different adults behave, observe their degrees of happiness, and decide who to emulate. Hopefully, he will choose to follow the examples of the wise, loving adults. The fruitage of different life choices becomes obvious to a teen. He really will want to be happy like the mature, Christian parent.
Specifically, tell Bible stories, watch children's Bible videos, sing children's Bible songs, speak of the Lord's blessings daily, and love your child tenderly. Deut. 6:5-7, "Love the LORD your God with all your heart, with all your soul, and with all your strength. These words that I am giving you today are to be in your heart. Repeat them to your children. Talk about the when you sit in your house and when you walk along the road, when you lie down and when you get up." All these behaviors will draw the child toward God. If the child may say, "There is no god," you might respond with a smile, "Yes, I know people say this, but God answers my prayers, so I know He's real." Then drop it. Don't argue. Just share your walk with God. Your life will be the real witness.
If the child struggles with anxiety, sooth him with your love. Wrap your arms around him and say, "Don't worry. Daddy's here. I will protect you. And God will protect us all." No heavy theology. No rebuke like, "Stop being worried. Don't you remember I told you God loves you?" Just calm, tranquil reassurance. All is well. God is at the helm. The child will feel your peace and be comforted. And one day, your peace will become his peace, because he has internalized your reliance on God.
It hurts to watch children suffer with unknowing. We would rather they simply believed our words. But the struggle produces the growth. It is the struggling with our own pain and suffering that opens our hearts to God. Your child or grandchild will learn of God when he learns God will always help him.