If the 'guardian' is someone other than the parent and is refusing for the minor, the 'guardian' signs as well as an LEO witness and everything is thoroughly documented. If we can get ahold of the parents, all the better - signed off as 'parents contacted, refusal by phone.' I have yet to have to use the 'protective custody "clause"' on a severely injured (or potential for) minor. The 'guardian' is usually more then willing to have the minor transported if they are complaining of pain/injury.
Also, here, only 18yoa (emancipated, military, pregnant (but only with regards to the pregnancy and related issues)) and older can 'legally' refuse of their own accord. However, I have 'refused' older minors (usually 16 or 17yoa) if the incident was relatively minor and they understand the gravity of the situation and potentials. As for in the hospitals, parents/guardians must sign all consents for minors.