Upvote:2
Under German law, legal guardians are allowed and required to supervise minor children. The extent of this supervision depends on the age of the minor, it is possible in principle that parents allow minors to travel to another city unaccompanied.
On a page which seems mostly concerned with German minors leaving the Schengen Area, the German Federal Police advises to bring a permission letter by the guardian/s (by both if there are two of them) which lists:
This is supposed to simplify their checks to avoid child abduction by only one of the guardians or truancy by the children.
This eu site also mentions regarding EU nationals that such a letter should also:
You should also be able to explain how you will pay for your subsistence while in Germany (not necessarily with cash in your pocket) and how you will travel on. Bring your passport (and for those who require it your visa), too.