What should I do regarding what I stole from the grocery store and from one of my relatives when I was young, knowing that I do not possess those items in order to return them to them?

If a boy steals money, he does not sin because he is not held accountable, but he must guarantee its return from his own money. The repentant thief must return the money to its owners or give it as charity on their behalf if he cannot reach them. He may also return the money in a way that does not embarrass him.