A room in a shared flat in Berlin is usually like €450-750. In Prenzlauer Berg, it's probably more like €700-900, but since this is a house with small kids and another renters and there's no mention of the room size so I will assume small and maybe even unfurnished, so let's say €700 would be appropriate. That means about €3 per hour for the babysitting, when Germany's minimum wage is €13.90 per hour and a babysitter would probably charge at least €20.