I really can't see those prices as accurate as a box for 4 people runs you around 380,00 yen. But hey, good luck !

They are accurate. Unreserved seats are 2,100 yen for adults and only 200 yen for kids between ages 4 and 15.

Disney - Are you trying to do both in one day? Don't do it!
You might be in line for 2hrs at EACH ride(that's if school is out)
I always recommend DisneySEA:D

Kimono - In Shibuya you can find some for 5000¥. I think the area is Dogenzaka. Sometime ago, I found a small shop selling Yukata, kimono and accessories.

Shopping - Visit 109 in Shibuya but don't buy anything. A lot of the items are sold cheaper elsewhere. :/

Your Brother - may enjoy Akihabara. There's an arcade called SEGA WORLD(CLUB) with tons of games. He might be into the Project Diva(favorite) or Gundam games. Akihabara is also known as electric city and has tons of surprises. :)

Parks - You can visit several parks and shrines in Kamakura, even the Daibutsu(Big Buddha) There's also one around the corner from Tokyo Dome. :)

Sumo - If you attend tournaments in Ryogoku, there's a museum(O-Edo Museum) nearby. :)

Some of your inquires can also be answered here:
:pandahurray: Visit Japan

masaegu: I stand corrected

Edo Museum is worth a look as they do have english labels on most things, only thing is the collection abruptly ends circa 1950-60's.

Very cool fire fighting kimonos though