I think it warrants a school visit and an open day to get a feel for what ISF is - firstly it is NOT an international school ( it says so itself). There is a lot of emphasis in Chinese and the cultural aspects, and I think honestly if the parents are not well versed in chinese or have some sort of strong chinese background, it may be tough for the kid to get through. I notice that westerners are def not the majority there. you also start with 70% chinese /putonghua from foundation onwards and then transition into English later, which means it would be hard for you to trasfer out to another school because the bi-lingual cirriculum is so diff - so if you are in for ISF you had better be there for the long term......
whereas I think for SIS it is still an international school with a bit more emphasis on chinese ( compared to other int schools) but still quite diff in extent from ISF.