From Briefings on HIPAA
It depends on the specific scenario. The child’s personal representative is the individual who has authority, under the HIPAA privacy rule, to exercise the child’s rights with respect to his or her PHI. This includes the right to request an amendment to the child’s medical record.
Generally, a child’s personal representative is either a parent or a legal guardian (if the court has appointed one). HIPAA does not consider a step-parent to be the child’s personal representative unless he or she has legally adopted the child.