To clarify and correct my previous post:
I found some additional information in an E/M practice book and went though all of the office visit examples. In every single one, if the parent is the only person who provides information, then you would not count it into the MDM. However, if the patient provides some information and the parent provides some information, THEN you can count the parent's information into the MDM.
There was no distinction as to the age of the patient, but using common sense, if a patient can provide relevant/accurate information, then they're old enough. This logic then also covers what to do when a patient cannot contribute to the office visit, such as if they're mentally incapable. The documentation would have to state the reason the patient couldn't provide information in such a case, but you could not apply the history from the caregiver to the MDM.
Does that help at all?