Greta's caretaker,
Luisa Neubauer, is connected with the German radical Left (I pointed the link to an earlier, pre-scrubbed, version of her wiki page). Greta's Antifa-supporting parents probably met her through leftist contacts.
Luisa Neubauer is openly committed to socialism and violence (Google translate of German article)
Searching for Luisa Neubauer antifa turns up some interesting stuff.