Personal opinion: most parents want their kids to read to improve english. but they do not know that language is a habitual learning... don't focus just on grammar, don't expect foreseeable result, just provide them with an environment that contact with english is necessary. make reading a habit, an interest, not a tool.
books wise, ask the kid what kind of story he/she prefer. and also ask yourself what kind of people are your kid comparing with? normal HK-er? if yes she don't have to read anymore. My exp was that singaporean kids are reading Jeffrey Archer at the same age. and they read for interest. so they don't feel pressure.