What should I do to atone for the sin of stealing pens and candy from a store when I was young, knowing that I do not know the owner of the store, nor whether he is alive or not?

It is obligatory to return what was stolen or its value to its owners, with due diligence if the exact amount is unknown.

You must search for the owners of the store to return their rights to them, or to their heirs if they have died, without needing to inform them of the theft. It is sufficient to return the right by any means.

If it is impossible to reach the store owners or their heirs after making an effort, then it is permissible to give charity equal to the value of the stolen goods on their behalf.