HK $ 40,000 per month could be tight, and first and foremost you will have to check out the rents. Obviously, location and size of apartments will dictate your rent, so it's a question of proximity to your work and schools (if applicable). Cheaper residential areas tend to be farther from city and will involve travel by either train or bus.
Also, apartments in HK tend to be very small, so in view of your family, you will have to look for a development that offers some sort of children's play area etc.
My guess is that unless you can live in an area such as Fanling, you will end up paying nearly half of your income on rent.
Schools are not cheap in HK either.
Personally, I would try and negotiate a deal that at least sees your prospective employer pick up the rent separately.