Plenty of people in HK live reasonably well on a lot less than $500k a month, so you should be fine, especially if you are getting help with school fees and accommodation.
We moved from London to HK last year and here are some observations based on your questions and our experiences:
The cost of living for an expat in HK can vary widely depending on what kind of lifestyle you want to lead. You can live very comfortably for very little, but you can find costs very quickly rack up if you want to pay for extra activities for your children, join a private club, etc.
Don't under estimate the impact of the heat and humidity. Although we are now reasonably acclimatised, a 15 minute walk to pick up a few groceries and a paper, which we thought nothing of doing in the UK, becomes much more of a chore in the height of summer here.
Cycling for commuting purposes is very unusual. Firstly due to the heat (and shower facilities at work seem less common here), but also because the roads are narrow and it isn't very safe as drivers just aren't used to looking out for cyclists and in any case taxis and public transport is cheap and frequent. We left our bikes in the UK, but in hindsight should have brought them with us as there are a lot of good trails suitable for cycling for leisure.
Schooling can be the most stressful part of the move, though it may be easier in NT than HK Island. Often parents don't give much notice to the schools that they are leaving, so the schools often aren't able to confirm places until the last minute which often leaves parents looking for places juggling offers - sometimes at significant expense as the registration fees are often non-refundable and you may wish to register at several schools. If your employer offers funded debentures, then take up their offer as that will greatly reduce the hassle.
Cash outflow when you first arrive can be very heavy. Just in accommodation alone, in the first 6 weeks we arrived, we had to pay one month deposit on our serviced apartment plus two months rent as well as two months deposit on our long term apartment, plus one month rent in advance and half a month agency fee, so try to get your employer to take care of the deposits as well as the rent.
As others have said, it is really important that your wife is supportive of the move. The first few months are much harder on the trailing spouse than the sponsored spouse, as you will get social interaction very quickly through work. However, as you have young children, she should find it easier to make friends via the parents of playmates than if your children were older.
Hope the move goes well.