What is the ruling on one who touches a boy's buttocks from behind a barrier?

If the touching is with desire, then the perpetrator is sinful and repentance is incumbent upon him. This is part of the fornication mentioned in the Hadith: "And the fornication of the hand is touching." The ugliness of the act intensifies if the touching is of a child, and it becomes even more reprehensible if the toucher is male.