We just came back from Singapore a few days ago (spending our Christmas there) There is a very big book store (the famous Japanese bookstore) at the Takashimaya (the Japanese Dept store) by the name of Kinokuniya. We bought lots of books there (spend close to HKD1500) at the bookstore. There are lots of activities books, games book for all ages. Definite a good spot to pick up books for kids of all ages (or even adults) I bought 3 for myself as well. My husband got a VIP card and got 20% off during Christmas period so is really cheaper than HK.
They offered a lot more activities books than those book stores in HK. The variety offered are wider, better and most of them are fun. For sure you can find one for your kid.