Typically a child will believe in their families faith if you can help them understand why the faith is substantive and what is the basis of your faith. Children today want to know why, not simply do it because you said so.
Having said that, children should be trained in the way they should go, not given the option to be trained. Left to their own devices, they won’t eat properly, clean up after themselves and host of other disciplines, their faith is no different.
Genesis 18:18-19
18 Seeing that Abraham shall surely become a great and mighty nation, and all the nations of the earth shall be blessed in him?
19 For I know him, that he will command his children and his household after him, and they shall keep the way of the Lord, to do justice and judgment; that the Lordmay bring upon Abraham that which he hath spoken of him.
Proverbs 22:6
6 Train up a child in the way he should go: and when he is old, he will not depart from it.