It depends what the children are into and if they have electronic gadgets they usually travel with. The nearly 2 year old will be the trickiest, I travelled to NZ and back with my DS at this age. If they have a seat for the nearly 2 YO, I would strongly recommend taking a car seat as that will keep them at the right height to use the tray table and securely fastened as a toddler who wants to run in the middle of a flight full of sleeping passengers can easily escape a lap belt! Hard for toddlers to sleep in reclined adult seat, a reclining car seat would be far better. Different ages for skycots/basinettes on different airlines, my DS was two old for one on the way out to NZ with one airline but qualified with another airline. Even if they're a bit big to sleep in it, it's a useful places to store all their bits and pieces.
Lots of preparation for the children, explaining about how they will be travelling for a long time and how they will watch movies on the plane and sleep and eat and sleep and play and SLEEP! I always give my DS a quiz on the safety demonstration, just to get him to pay attention and stop disrupting others at this time.
I could recommend a magna doodle as good entertainment, the 'pen' is attached with string so can't disappear down the back or under the seat, can't draw on the seats either.
Child sized headphones and a plane adapter for the inflight entertainment system.
Via LA gives them a greater luggage allowance than Hong Kong.
A beach ball is great for chasing around the departure lounge to give them some exercise before the flight, if possible go to an empty gate area next to their departure gate so they have some space to run.