Lots of nice beaches in HK - on the Island there's Shek O, Big Wave, Stanley (combine with the market there) and Deep water Bay. Lots of the outlying islands also have good beaches and are easy to get to by ferry - try Cheung Chau and Lamma. Good list here - http://www.theguardian.com/travel/20...ches-hong-kong

Swimming pools - I think the best outdoor ones at the hotels we've stayed at are the Island Shangri-La and the JW Marriott both at Admiralty. The Island S-La has the biggest regular rooms in HK from what I remember - your son can stay free of charge - under 18s stay free which is a big bonus - some rooms in other hotels would be too small to take an extra bed.

Oh and if you want to splash out a bit the IC has the Presidential Suite with its own infinity pool facing HK harbour!