Since children can't provide their own informed consent, its very morally dubious to give them medical treatments for something that is unlikely to make them sick, and seems on balance for the benefit of the parents. This is quite different from the other vaccines they currently take.

In the event that the covid vaccine turn out to have long term health effects, a child has a long life to live with those side effects, unlike a 70 year old. So their risk is greater, and their potential suffering much longer lasting.

Societies have generally made sacrifices for their children, and not sacrificed their children for the benefit of the elderly. The courts have also generally held the interests of the child supersedes the interests of the parents.