You can now find many English books at Kinokuniya Book Store in Shinjuku!
You should really go,because not many bookstores offer English books that fit kid’s ages.
I always find adult sized books or kid books,but never books for 10-15 year olds.
But kinokuniya Book Store is different,so you might find some new books there!
For more info,go to http://www.kinokuniya.co.jp/c/store/Shinjuku-South-Store/20140526120050.html