Ropes tree climbing courses such as Kletternwald-Neroberg, in Wiesbaden are very popular in Germany. At Kletternwald visitors purchase a 3 hour ticket. After a 20 minute safety course climbers explore the routes through the trees labeled by difficulty like ski runs. Our very small seven year old was able to fully participate in the outing as are children over the age of six. Since she was small, she did have trouble reaching a few things, so it was necessary for her turn to be between two adults. Children over age four can participate in a special children's climbing route.
Fun Forest near Homburg offers a similar climbing adventure, but since our daughter was 7 and and the Fun Forest had an age cut-off of 8 years for many of the activities, we drove the extra half-hour to Wiesbaden.