The argument that a child is
unable to give informed consentis probably the most popular argument used against sexual relations between adults and children. And one of the most hypocritical, because there is no other domain of life where anybody bothers about consent of children, informed or not.
Nonetheless, let's consider the argumentation. Children may like to do things which may be harmful for them or others, because they are not mature enough to consider the possible consequences. Thus, to protect children from harm, it is sometimes necessary to forbid something they like. As examples, usually drugs or car driving will be used.
The flaw in this argument is obvious:
Consensual sex is not dangerous, but healthy.In the worst case, we have to assume some probability of harm caused by accident. This makes sex more comparable with sports like skiing instead of unhealthy things like drug misuse.