First let me say this is not "migrant children". These are men primarily aged 15 to 24 who have been causing significant problems in the train station. As noted in the article these men "spend their days drinking, stealing, and accosting women".
I can understand why they Swedes are upset by this and by the reality that the police make no proper efforts to control the situation and make the station safe.
However broadly going down to the station assaulting anyone of foreign origin is wrong. I can understand taking direct physical action if an individual migrant is caught stealing or assaulting a women --- and then turning over the perp to the police.
The better path here for the Swedes is to publicly demand immediate law enforcement action to clean up this station and other locations. Make large pubic protests demanding action that can not be ignored by government officials.