Our children's life in HK (southside) has been broadly similar to being in Singapore.
We are in a condo with extensive facilities and their everyday routine (school - playground/pool) did not change much so in that repect not much of an upheaval.
a few things that are different in or experience:
On the whole, school are vastly better in Singapore. We are still reeling from the trauma of finding suitable school places in HK. 3 years on, we are generally disappointed by the standards in HK and for that essential matter would definitely consider going back to HK.
In terms of after school activities, its pretty much the same, all readily available.
Mandarin tuition is much better in HK though and DC get more opportunities to practice. There is a sense that english is NOT the dominant/native language and it really pushes them (and us) to make an effort. The proximity to China is more marked so a more interesting experience than sterile Singapore.
In Singapore, its a status thing and locals only speak (very bad) english/singlish.
Lots of flat land to run around in Singapore for sure but you need to get used to the constant weight of the heat and humidity. Its really different from HK as its relentless. You'll be in the pool most of your waking time when kids are not in school or doing an air conditioned activity. Which is lovely but not a patch on having seasons and hiking/running/tennis/sailing in crisp air.
The scenery is also very different. HK is stunning for the contrast between sea and mountain. In singapore, you hardly remember you are on island as there is no such dramatic reminder, the beaches are totally a waste of time. But the lush vegetation makes any walk really worthwhile.
Life in Singapore goes at a different pace. Its an bubble caught in a tropical time warp. It really feels like you are on holiday all the time (even for working parents). For me it was a bit unsettling after a few years. I did get Island fever. It all gets a bit incestuous and you cant take a step outside without bumping into someone.
I find adults much happier in HK, fast pace in town to work/play/discover vs chilling on the beaches, hiking/sailing. The best of both worlds.
I also meet much more interesting people in HK. Some are roughing it up, some have fab expat packages. Better mix in terms of age groups and background/industry. You can feel the nitty gritty of being in a big city in HK. I get the feel that you are in an inspiring environment which gives you a lot of energy to set out and learn something new or develop on a professional level should you be so inclined.
Singapore is pretty dull in comparison and increasingly difficult to work with a dependant visa.
I think the kids also benefit from seing a bit of dirt in the street and walking around old local neighborhoods (kennedy town, wanchai, aberdeen). No such exposure to "real life" in Singapore which had made mine very disconnected from the "normal" life.
People generally more cliquey in Singapore with dividing lines roughly around nationality/ethnicity & schools (same as HK but more marked).
The school mum scene is hugely important in Singapore as much less trailing wives work. So its a usually a very welcoming close nit pack which makes our children's life great.
Let me know if you want any more specifics. Hope I sound balanced on the whole. We love both but now prefer HK. Better suited to our life goals now. We loved being in Sg with very small children (and a new baby) it was the best place to be for us at that stage in our life.
HTH