Sins that are between a man and his fellow man are not forgiven on Yom Kippur unless he has appeased him (Yoma 85b).
One Sabbath day, the aged Steipler Gaon insisted on going to a particular synagogue some distance away. His family tried to dissuade him because the long walk would be too taxing, but he insisted and in fact made the difficult walk.
The Gaon later explained that some time earlier, he had reprimanded a young boy for putting a volume of the Talmud into the bookcase upside down, which is considered to be disrespectful handling of a sacred book. The boy then showed the Gaon that the volume was bound incorrectly; the cover was upside down, but the book itself was put away upright. The Gaon then apologized to the young boy.
"But because this young boy was not yet bar mitzvah," the Gaon explained, "he was a minor who was unable (according to Jewish law) to grant forgiveness. When I heard that he was to become bar mitzvah this Sabbath, I had to avail myself of the opportunity to obtain proper forgiveness."
Everyone at some point says or does something that offends another person. Too often, we dismiss the incident without giving it a second thought and so are unlikely to remember it so that we will apologize when the opportunity arises. The above incident should help us realize the seriousness of offending a child, and the importance of obtaining proper forgiveness.