The National Children's Castle or Kodomo no Shiro is another place just for children. It's very close to the Jido kaikan, and near the Omotesando mall. This place has a number floors also with lots of things to do--workshops, computers, book and video libraries, open playgrounds, large halls, theaters and more.
The kids spent most of the time playing with two expat kids in the mini-kitchen on the third floor while their moms chatted away with each other. Across the hall on the same floor is the Fine Arts Studio. Here again they spent a lot of time painting on this long white board with different colors of poster paint. I thought this idea was ingenious--a very simple idea, but with maximum impact on the kids. Imagine the freedom of being able to splatter all kinds of paint on a wall--they were thrilled.
Also in the same room was the 'project-of-the-day', creating simple masks. Behind this creative room was a room full of computers with video games. We spent our last remaining minutes here until closing time, leaving still a big area of the Children's Castle unexplored.
When we talk about Japan, the kids mention the Jido kaikan and the Children's Castle (aside from Disneysea of course). I know they will always have good memories of Japan, having experienced special places created specifically for kids.