Can a child under the age of 18 refuse medical treatment ordered by a doctor?

Depends on context. I guess i would argue that it is a matter of degree. Do the parents desire the final treatment? They get the final say. Most people would try to negotiate with the child. If it is something minor, it may be worth trying something doing. If it is critical (e.g. Emergency treatment, chemotherapy) i would argue that the child will need to follow the doctor's orders.