The sefer Minhagei Vermeiza (Worms) explains that children do not attend a parent’s second marriage. This is because the children’s presence is a reminder of the first spouse, and could thus mar the joy of the occasion.
Rabbi Yosef Shalom Elyashiv, shlita, was asked about this custom. He gave the following explanation: Children have an obligation to respect their parents even after the parents have passed away. Attending the marriage of a surviving parent would be disrespectful to the deceased parent.