The bookstore in Lee Garden is awesome. I have to pull my daughter from that store every time - she doesn't want to leave!
There is also Page One in Times Square, Causeway Bay. Both my kids love their children's section.
Bookazine is not too bad, but it depends on the store as they vary so much in size and content. The one in Princes Building is a good size and their kids section is decent.
There is also Metro Books. I've only been to their store on Gloucester St in Causeway Bay. It had some titles I hadn't seen in places like Dymocks or Page One.
Cosmo Books english section is not too bad either although not huge - on the first floor of their store on Johnston Road in Wan Chai. Their stationary section is quite good though - lots of art and craft stuff at reasonable prices.
I agree with Nicole though - more often than not it is cheaper to shop online for the books unfortunately. I love bookstores, but with a place like Dymocks it is very hard to justify how much you spend there when it is so much cheaper online.