The humidity is going to be a lot worse than anything you've probably experienced, and you will get several months of it. You'll also have a wet cold in the winter, but you are probably used to that.
Yes, there is green areas, but finding a 'house' with a garden will probably require you to look at someplace like Discovery Bay as most of the places on the island are a bit smaller or apartments.
It is very easy to meet other expats. There is a whole 'we're all 6,000 miles from home together' so you have an instant similarity.
Having kids in HK is fun, you'll have a nanny no doubt which will help a lot. The one thing that does worry me is the pollution. The air is a bit much on young lungs.
HK can get a bit claustrophobic though. It is a small place, even more so if you live the expat life in which you are one of about 100,000 or so (and everyone knows everyone). I hope you budget for some travel every now and then as you will want to get away every now and then.