There's a notion that the yetzer tov enters the body at the age of bar mitzvah. If this is so how do we see children that behave properly and act kindly towards others?
Rabbi Simcha Wasserman zt'l said (not exact quote. see "Reb Simcha Speaks"): "children are like new immigrants which arrive at a new country. The immigrants quickly adopt the customs of the local population. So too children quickly adopt the behavior of their parents..."
this answer does not explain all cases but i think it explains many.