We visited Berlin with my daughter (aged 2), me (aged 30-something) and my parents (a bit older). Location of the flat was great, close to the U-bahn, Tempelhof Freiheit and and Hasenheide park. Me and the kid didn't mind the 6-story climb as we live on the 5th floor without elevator anyway back home in Helsinki.

Flat is not completely childproof as the child proved, but it was just a matter of lifting breakables and housegold chemicals higher than 120 cm :) Ultimately: the flat is great, better than a hotel for travellers with kids, but every parent is ultimately responsible for their own, so don't expect that you can just open the door and let the small people roam free.

All in all, a very good stay, and also the Grandparents agree wholeheartedly. Nina was a great host who we didn't get to meet but carefully to warned us if she felt she might have to swing by.