We stayed in Nadja's apartment with our three kids (ages 3-9) for a week and had a wonderful time. The appartment is spacious and nicely decorated. It's in a modern residential area, and has a small garden and a public playground right next door. Our kids were absolutely thrilled with all the toys in the apartment.
Köpenick is a nice part of Berlin, and it is fairly easy to get around by public transport. We would definitely stay there again in the future.