Welcome to the forum.
1. Where is the best suburb for us to go?
I define the best as follow for my case:
- cheap enough(100~300$/week),
- good suburb for our 5 years old kid which is supposed to go to school when we arrive there,
- not too far from Sydney University (Cumberland Campus),
Sorry, can't help with recommendations on Sydney suburbs as I live in Victoria.
Have a look at realestate.com.au for rental properties. On a quick look, you're looking at around $350-$450 per week in Lidcombe (Sydney is not cheap).
Moving to Sydney or Melbourne? Find where to live with Cityhobo - Home
is a good place to get general info on suburbs
Child Care Services Directories - City of Sydney
Child Care Centre Search - CareforKids.com.au ®
2. If schools are working from 9~3pm and both of us are working till 5~6 PM what can we do with our 5 years old kid from 3~5PM everyday?
3. Is it better to shift our furniture from Malaysia to there or it's cheaper to buy everything there?
It's personal choice if you leave most of your stuff behind and buy new here. We decided to bring over pretty much everything. To us it seemed more cost-effective.
4. I think the cheapest airfare from Malaysia is JetStar with around 900RM ~= +300 USD. Do you know any other cheaper option since I may have to return for second time to Kuala Lumpur for something to finish.
Don't think you'll get any cheaper than JetStar. Sometimes airlines do 'specials'. Have you checked any airline websites?
Try to do as much research as possible before you move over.
If you have any other questions, just ask.