My son is in the local stream at KCS. Although both my husband and I can speak Mandarin, we do not speak to him in Mandarin, and do not reinforce his Mandarin other than by playing the CD of songs the teacher gave him at the start of the school year when he asks. In spite of that, he is doing brilliantly. Right now he is in K2 and recognises many characters. He is learning to write some characters also. There is a half-hour English lesson every day, and they seem to do phonics. He is learning to write letters and numbers in addition to his Chinese characters.
Most, but not all communication, is bilingual. The teachers seem to be able to speak English, Mandarin and Cantonese, to varying degrees.
English is currently his dominant language because my husband speaks to him exclusively in English, along with our helper, neighbours and most friends. Note that in spite of being in the local stream, the children seem to default to English when they speak to each other. At least, this is what I notice on the school bus.
We are all very happy at KCS and enjoy being part of an established, relatively well-organised school. Our only real complaint is that the snacks are not healthy enough for us, but we make up for that by packing a snack for our son. The only 'growing pains' related to us: we sometimes find the system bureaucratic and inflexible.
My son was in Woodlands Montessori Repulse Bay and we were all SO unhappy. My son was not happy with the lack of structure (although his first educational experience was at a Montessori school) and we were not happy with the poor organisation and administration.
As for IMS, I have friends whose children went there and they also have been very happy with the system -- 1 English native speaker and 1 Mandarin speaker taking turns leading lessons, etc. After three years at IMS, one of the students has gone onto another English-medium school and has qualified for its native-level Mandarin class. It seems to be a smaller school and there seems to be more of a community. KCS is huge.
Good luck with your decision. We love KCS.