When our kids were growing up, my wife always had Christian music or preaching and teaching cassettes and, later, CDs playing all day long in the house and in the car. Our kids had it whether they wanted it or not. They also had the example of an amazing woman of God who showed God's love to them. And it was all of that that brought them back to the Lord when they had each strayed off deep into Satan's realm.
My Parents were believers, but going to church every Sunday wasn't their top priority.
They made sure we were Baptized, had Bibles and hymnals, got Confirmed, and went to Sunday School (when they went to church). Sometimes, Mom took us and went to church by herself (or took me/us as we reached Confirmation age). Dad slept in on some of those mornings (quite possibly because of late musician hours and extra busy week or coming off of tour).
They also gave us big Bible story books, which they would read to us before bed when we were very small.
We also prayed before dinner every evening (family dinner every night was mandatory; once a week away if dating age or for school function).
The big thing helped me was my Maternal Grandmother terrorizing me with Hell and then teaching The Gospel.
She pulled absolutely no punches about Hell (fire, brimstone, lake of fire, utter darkness, stench, worm that never dies, wailing and gnashing of teeth, separation from God, etc. forever and ever and ever), and when she taught me the Now I Lay Me Down to Sleep prayer and what it really means, as a 2 or 3 year-old.
I was blessed with saving faith at 3, right before or after that, in Molly's Sunday School class
Grandma also used to ask me, "Wouldn't it be terrible if Jesus came back right now and found [whatever transgression I had just done or not done]?!!!"
Old fashioned, stern, and strict, but absolutely the right way for me
Never too early for a child to learn the fear of The Lord, Hell, and The Gospel
Really learning the 10 Commandments came afterwards (for me)
Something to consider: It's a lot easier to teach a very, very young child truth because he or she absolutely trusts and believes what Parents tell him or her, and the questions are clarifying ones, not challenging authority/knowledge ones. There's no world gobbledygook to undo or world influences to counter. They're little sponges, soaking everything up

This is a big part of why TPTB want as many kids in pre-school as possible, so
THEY can determine what the little kids learn without question, and NOT the parents
