"for sure a kid have less sins on his/her "karma" and so is purer than an adult"

If you think of sinfulness as an accounting system by which we are punished for what we have done, yes.

If you think of sinfulness as the condition of being headed in the wrong direction, quite the opposite. Those who are capable of learning tend to improve their behavior as they age, as they practice self-control and better understand how they should act.