We call them secret playgrounds because we hardly ever see them labeled on a map. Tucked between buildings and across from cafes, these wild spaces are a blessing to traveling families. Berlin is the only city where my kids come back to the hotel every night with dirty feet, a collection of sticks, and sand in their pockets.  Secret playgrounds are sometimes simple with a few swings and a wooden structure, but they can also be elaborate castles or ships that turn into dragons. In Berlin, we also spend plenty of time at “official” playgrounds playing soccer. These places are the reason our children ask to go back to Berlin.