Sin

I have 2 young kids. We live in a fallen world. We are sinners from birth. How do I teach them about sin without it becoming legalism?

It's easy right now - I just say, "Don't touch that!" or "Stop hitting your sister!" and then there are consequences for not listening or disobeying.

What happens when they start asking me why it is wrong?

"Because you will be punished"
"Because it's not nice"
"Because society doesn't condone that osrt of behavior"
"Because I say so"
"Because God says so"
"Because it may have unforseen consequences later on down the road"
"Because the Bible says we shouldn't"
"Because my church says we shouldn't"

I don't think any of these communicate with the heart.

I have all sorts of fancy intellectual arguments I can spring for them. And I can turn to my reference books on systematic theology and have them read pages and pages about God's holiness and our sinfulness.

But, how in the world do I reach their hearts?

How do I convey to them the idea that sin is more than a long list of "thou shall nots"?