Source is the police, here is the Link to the news ticker
Here’s the relevant part as a screenshot:
Gegen 18 Uhr wurden Einsatzkräfte auf einen Jungen aufmerksam, der zuvor offenbar ohne Begleitung an der Versammlung teilgenommen hatte. Aufgrund der noch andauernden polizeilichen Maßnahmen im Nachgang der Versammlung wurde der Elfjährige zu seinem eigenen Schutz in Obhut genommen. Sein Vater wurde benachrichtigt, der ihn circa 90 Minuten später abholte.
Translated by me:
At about 6:00 PM officers noticed a boy attending the event clearly unsupervised. Because of police activity after the event the 11-year-old was taken into police protection. His father was notified and he took the boy with him after 90 minutes.
In the video you can hear them yelling “Hey! Release the kid!” and stuff like that. They also call the police “murderers” for protecting the child against the thrown stuff, so I don’t really take them seriously.
I haven’t found any statement from the father, or someone adjacent to them, so I don’t know what they have to say about this. Maybe someone can find something.