Just found this news at the german police report and like to share the info:
to make a long german story short: mono-wheels are seen by german low as a vehicle and to as a toy.
so that have the meaning: you have to have a license (what kind, legaloffice might know), you can not use it in the pedestrian zone and other areas. And you have to have an insurance for the vihicule.
Just like to send this info, cause it sounds, like get easy in trouble, just because of "unexpected" special law over here.
As well importend, if your kid got something like that... not talking about an exident yet (might get very tricky then....)