My former Sunday school students who attended FIS lived in Happy Valley, but the apartments there do tend to be a bit smaller and the buildings taller. Shiu Fai Terrace in Wanchai is up on the hill off Stubbs Road, which is also within an easy distance to FIS by public transport. The buildings there are older, and therefore the units tend to be larger, but also the buildings are not as tall - ~ 12 stories. From there, you can walk up to Bowen Road, which is a nice paved road with great views of the northern side of the island. There's also a children's playground just down the hill. You would also be within walking distance of the nearest Maternal and Child Health Clinic, from which pregnant women and pre-school children can get all their ante-natal visits, scans, tests, vaccinations, etc. for free. It is also close to shops, supermarkets and wet markets along Queen's Road East and you can take minibus 24M back directly in front of your doorstep!
For schools, if you're pretty confident that the children will eventually be going to the french stream at the French International School, you might want to try some local schools. Just up the hill on Stubbs Road is Ling Nam Kindergarten, a lovely child-centered school which teaches in both Cantonese and English. Or if you're keen to immerse the child in learning Mandarin, I would recommend Kiangsu Chekiang School, a local independent school, which teaches all their subjects in Mandarin except for their English class. There are quite a few child of western/mixed culture backgrounds there.
The Alliance Francaise is now in Wanchai.
Welcome to Hong Kong and enjoy!