You've been given great advice on your options for an earlier flight. Let me give you another perspective. While space will almost certainly open up in the next six months for this route, haven a 15-hour layover in Hong Kong is not such a bad thing. It's a decent way to break up the travel, especially with young children. I'm not sure what your timings are, but you could get a day room at a hotel and have a snooze (briefly, though you'll likely have slept on the flight over) and then venture into Central to get your body clocks adjusted. Have brunch, go up to the Peak, take a swim at at the beach in Stanley, and then back to the airport. There are bigger problems.