Does Hong Kong have a hotel in the airport (like Singapore)? Reason I ask is that having flown internationally with a 10 mth old and then a 20 mth old, we chose a few hours (5? Can't remember exactly) stopover the second time purely so we could get a few hours sleep in between flights. The airport hotel in Singapore you don't even have to go through customs etc and can hire a room by the hour. My DS never slept well (or barely at all) on noisy planes even though we always had a bassinet. And the stopover allowed us all some much needed zzzz before attempting the second half. It was lifesaving. I would always travel this way if possible.
Failing that there is always phenergen (shoot me down for mentioning that one!).