What should I do regarding my taking things that don't belong to me from friends and siblings in my childhood without returning them, and taking sweets from the grocer secretly and without paying?

A child is not held accountable for their actions before puberty, based on the hadith: "The pen has been lifted from three... from the child until they reach puberty." However, they are liable for what they destroy or take from others' property, in accordance with the hadith: "On the hand is what it has taken until it returns it."

If the property is present, they must return it. Otherwise, they must return its equivalent or its value on the day it was taken. If the owner or the amount is unknown, they should donate it on their behalf as a precaution, or spend it for the benefit of the Muslims, as stated by Sheikh al-Islam Ibn Taymiyyah.