How is this reasonable, they're traumatizing the kid why exactly? 10 year olds are out on their own all the time in Germany and aren't being chased by police.

How is that reasonable? If you're protesting you shouldn't have to fear being chased by the police.

How dare you ruin this perfect moment of outrage with reason.

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.