Kiddyland is not a good store for this.
I bought one that came with some track in a convenience store at a train station in Osaka 7-8 years ago, which would be perfect for a 6 year old.
Toys r Us has a major presence in Japan. But stores are in more suburban locations. I think the best bet for a tourist is Odaiba, as mentioned above.
http://www2.toysrus.co.jp/store/en/kanto2.html#tokyo
I shopped for toys at Donki last week and can't say that I saw anything like this. I probably would have purchased one.