Hi there!
Re. carseats - I had the same question before we moved here. First of all you are not required to have a carseat in a taxi at all.
For your newborn, you will probably feel safest with an infant seat in the taxi. We use our old Graco seat from the USA and it is just fine with the seat belt through it. Also bear in mind that in a taxi you are rarely traveling at really dangerous high speeds. just about any seat with slots to run the seatbelt through should work fine in a taxi.
for our older daughter (4) we don't use a carseat of any kind in the taxi. We also sometimes just use a Bjorn or other carrier with the baby (8 months) in the taxi if we're going somewhere where it would be difficult to lug around both the carseat and the stroller.
of course if you choose to buy a car you can install the carseat as needed.
I was really freaked out about the vegetables and fruits and it has been fine. I try to buy different things from a variety of sources/countries/different stores to make sure I'm getting a variety of toxins. LOL. that's about it. There are also a couple of stores in Central (Three Sixty and Oliver's) that sell a range of organic fruits and veg. Much of it comes from Australia and New Zealand. Other things are from China but certainly not all of it.
In fact you'll see a lot of Australian brands and products here.
The water is fine, I used boiled tap water for my baby's formula when I need it. In Mainland China, this is not the case but in Hong Kong the water is pretty good. My baby dunks her mouth in the bath water all the time - she is a fast one - and so far no problems.
Kids' shopping - I have a hard time with clothes here, the sizes are different and the quality ranges from fabulous and expensive to cheap and you get what you pay for, so I tend to stock up when I am back in the States. But, there are good shops in Central, Causeway Bay, and Stanley Market has tons of kids clothes for sale as well. If you are like me you will have a bit of fun shopping around, learning the different shops, and finding what you like.
Oh and re helpers - yes, would definitely recommend - it is one of the best things about living here - lots of things take longer here, IMHO - running errands, doing the shopping - all take longer, not the same as the suburban/urban lifestyle I was used to in the USA and that I think is pretty similar to Oz. So having an extra pair of hands helps. Plus, who likes doing dishes or laundry? I still do most of the cooking as I enjoy it, and having an on-call babysitter is about the best thing ever. There are lots of resources out there on the interweb about how to find a good helper, conduct interviews, etc - if you have questions or would like some info PM me. That can help make sure you get the right person. And you know if you go through the process and it's not working out, you end the contract and find someone new. It happens all the time. It's a hassle and you'd have to start over and find a new person (and the hiring process and getting the helper their visa to work for you in HK can take about 6 weeks) but to get someone you are comfortable with it is worth it. When you get here start asking around for recommendations, join the AWA (American Women's Association (it's for all women)), and you'll probably find someone who comes well recommended.
Also recommend you check into the AWA and the YWCA - they run lots of programs for newcomers and are great for meeting people and learning your way around.
best of luck to you. living here (2+ years for us so far) has been a marvelous time for us.