Wawa,
Glad to see you doing research first. The answer is that the availability of Primary education is patchy. If you live on Discovery Bay then the only way to get into the International school there is to put the child's name down at birth, pay for the pre-school years out of your own pocket (Cathay doesn't pay) and then your child will get a place in P1. Cathay only pays for 90% of the cheapest education available (ESF) from age 5.
You will be amazed at the cost of decent education here. Budget for about 30,000HKD per year for kindergarten/Reception classes out of your own pocket. If you don't go to ESF (which isn't the ENGLISH education that you may think it is) then CX contribution remains at 90% of the ESF fees. This will leave a considerable shortfall.
One other school that hasn't been mentioned on the list is Kellett School.
www.kellettschool.com
Fees: Reception 66,000HKD pa
P1-P6 77,000HKD pa
Hope that this helps