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. “
That’s one way of looking at. The converse could also be true,however, that by attending, they are showing respect to the LIVING parent. I think it might be more that the CHILDREN themselves. as happy as they might be for a remarrying parent (if they are in fact happy), might be pained thinking of their own loss or in the case of a divorce, still harbor the wish that the parents were together, and it could be an inadvertent ayin hara for the new couple. Is this an actual halacha or a minhag that everyone accepts? I know I have been to remarriages where the children on one side did not attend, and the daughter on the other side did. In both cases, the kids were happy for the parents (both situations were divorces, where the ex-spouses were nice people).