What is the religious ruling on returning inexpensive items that were not returned to their owners after being borrowed or taken by mistake? And how can one be absolved from the rights of children who were beaten in their youth, after they have grown up and the one who beat them is no longer in contact with them?

The rights of people are protected, and it is not permissible to transgress against them. Therefore, what you borrowed must be returned or its owner must absolve you. Otherwise, you should donate its value as charity. Disciplining and educating children by striking them is permissible. If it was impermissible, then repentance for it is achieved by seeking absolution from those who were wronged after they have grown up, unless that would cause discord. Otherwise, repentance is achieved by seeking forgiveness from Allah, drawing closer to Him through good deeds, and giving charity on behalf of those you wronged.