Obstacle course for kids
This large park, the Heiwa no Mori Koen Field Athletic Course (peace forest park field athletic course, is located near Omori station on the Keihin Tohoku line (you can ride this line from Tokyo station).
The park is filled with fun obstacle course-type playgrounds that kids and adults alike can enjoy. Ziplines, rope swings, rope bridges, and slides are among the things you'll find here. I have always found that the playgrounds with these types of set ups in Japan tend to be a bit more dangerous, at least by standards back in the U.S. But these parks in Japan are so much more fun to just let loose on, and if you keep an eye on your kids, there's nothing to really worry about.
Here is a link to an article on the park: