Hi Jenni. I am British and have lived in Hong Kong since August 2009. I have one DS (16mo) who was born here, and another on the way, who will also be born here. We are here on an indefinite stay (local, not expat package).
So this post will be long but I'll try to answer your questions
First of all, re your work, what do you do? Depending on your skills, it may be really quite easy to get a job, or it may be next to impossible. The visa is not the problem (as a dependent you have the right to work). The problem is the language. If you can do something in the English speaking economy (banker, Big 4 accountant, lawyer, teacher, high management position in an international company etc) it's not that hard to find work. However, if you do something that involves contact with the whole population (eg nurse, police, social worker, lower level management etc) it's almost impossible because you need to be fluent in Cantonese.
Nurseries dont really exist. Everyone who works (and most people who don't) have helpers, who are live in nannies/housekeepers. They are usually from the Philippines or Indonesia (expats tend to favour Filipinas as their English is usually better). They have to live with you and work only for you (6 day week). In practice people do have live out or PT help but it's not legal. Helpers usually don't have any formal childcare qualifications, but many do an excellent job. Some of the younger ones are qualified nurses/teachers but IMO, this doesn't always translate to them being better nannies. The minimum wage is now around $3900 per month, plus either a food allowance of c $800 or you let them buy their food out of the housekeeping money. Most experienced helpers earn closer to $5000pm plus food. You also need to provide them with a room (most apartments have a designated helper room), and are responsible for their medical bills (can get insurance) and a flight home every year. Most expats give a month's salary at either Christmas or Chinese NY as a bonus. One issue in HK is that it almost becomes hard not to have a helper because there is no other source of babysitting so if you want to go out in the evening, you're a bit screwed unless you have a helper.
I knew no-one when I came to HK and I made friends easily, despite not having any children at that point. People are very friendly and open to making new friends/ talk to their neighbours. There are tonnes of activities for pre-schoolers and people are usually keen to arrange meet ups and playdates. You will find a high helper ratio at many of these activities, but often playgroups will have designated "parent" sessions, and there are a few parent only playgroups, especially on the south side of the island. I would say that from my experience, the vast majority of local (HK CHinese) mums work. Amongst the expats it's about 50/50.
Re having kids in HK. Well, as I said, my son was born here and I had a very positive experience. I went private as we had insurance through my DH's employer, so you'd need to check if that's included in your healthcare package. A private delivery (incl all antenatal care, an obstetrician led delivery and a private room at Matilda/Aventist/ Sanatorium) comes in at around HK$80-100,000. You can get this down to around $60,000 by opting for a shared room. Most people who don't have insurance use the public system which is adequate but not amazing. The usual hospital for expats is the Queen Mary on HK island (but you go to your nearest one- it's like a catchment system). QM is about as good as NHS to be honest. The medical care is very good- nothing bad will happen to you - but it is extreme "no frills" (take your own loo roll ) and they are behind the times re birthing positions and there are no waterbirths/home births etc and bf support is terrible. The wards are busy and noisy and visiting hours very limited (2 hrs a day or something). The nurses in the public hospitals on the island do speak good English. In the New Territories/ Kowloon maybe not so well. One option some of my friends have gone for is to hire a private midwife to act as a doula when they give birth in the public hospital.
Other things: Rents are astronomical. They currently run around 2-3x Central London, so apartments are small. What sounds like a really generous housing allowance may not be all that much in reality. The island is most expensive. Kowloon and the New Territories are much more affordable, but there are fewer expats. Off HK island, the expat hubs are Discovery Bay, Sai Kung and Clearwater Bay.
Offsetting rents is tax- 15% flat rate, so very low compared to the UK. You have to pay a year in advance though so painful the first year.
It's a lot greener than most people think. Lots of hiking trails, beaches etc. However, there's also bad air pollution.
HTH