if an animal kills a Jew it deserves the death penalty source
if a child (under 13/12) kills a Jew he is completely free (no consequences)?
if yes why?
The Ramban (BERAISHIS 9:5) says that an animal which kills a person does not receive death as a punishment because animals do not get rewarded or punished for their actions (as they have no Bechira (free will)). He goes on to say that it is a special Gezairas Hamelech decree from the King Hashem that an animal which kills a person gets stoned. A child which does not receive punishment for his actions and there is no specific decree that he is put to death therefore is not killed for killing another person.