Participation of a Young Orphan in a Wedding During the Year of Mourning

Can a young child in mourning within the thirty days of his mother's passing attend a wedding?

He may attend the wedding, as mourning does not apply to young children. See the source for the opinion of those who are stringent in this matter.

In the Gemara Moed Katan, page 14, it states: "Shmuel said: A young child born during the festival is permitted to have a haircut during the festival, as there is no greater imprisonment than this. During the festival - yes, initially - no. Rabbi Pinchas objected: All those who are permitted to have a haircut during the festival may have a haircut during the days of mourning. Those who are forbidden to have a haircut during the festival are forbidden to have a haircut during the days of mourning. And if you say there is a dispute regarding the child, then mourning would apply to the child. But it is taught: they tear the garment for a child due to emotional distress! - Rav Ashi said: Does it state that it is forbidden? Perhaps some are forbidden, and some are permitted. Ameimar, and some say Rav Shisha, son of Rav Idi, taught thus: Shmuel said: A child is permitted to have a haircut during the festival, whether born during the festival or initially. Rabbi Pinchas said: We also teach: All those who are permitted to have a haircut during the festival may have a haircut during the days of mourning. Those who are forbidden to have a haircut during the festival are forbidden to have a haircut during the days of mourning. If you say a child is forbidden, then mourning would apply to the child. And it is taught: they tear the garment for a child due to emotional distress. - Rav Ashi said: Does it state that it is forbidden? Perhaps some are forbidden, and some are permitted." It is explained that there is no prohibition to cut the child's hair during mourning, and they tear for him only due to emotional distress. This is also ruled by the Beit Yosef in Siman 389 in the name of the Laws of Joy, and similarly in Mordechai in the Laws of Mourning, end of Siman 499. The rabbis are divided on whether there is education in mourning for a child. According to the opinion of Ri"tz Gaiot and Taz and Bach, there is a law of education in mourning for a child. They hold that the law explained in the Gemara, that they tear due to emotional distress, applies to a child who has not reached the age of education, but for a child who has reached the age of education, they tear according to the law. However, in Shach, Siman 384, Subparagraph 5, it is written that there is no law of mourning for a child, and so it is written in Beit Hillel in Siman 389. In practice, they adhere to the main law, that there is no education for a child in mourning.