Outcome 1a: Child suffers for a while but then picks up Mandarin without much help and becomes fluent. In this case, the child is able to absorb and learn Chinese quickly because they are good classroom learners and can pick up the language in school.
Outcome 1b: Child is not able to pick up much Mandarin, and ends up not having a good experience in school. Either parents take them out of Chinese school halfway or child ends up being miserable in school. I have seen kids graduate Chinese school not knowing a word or refusing to read or speak Mandarin.
Outcome 2a: Child will struggle to understand what is being taught and has a steep learning curve. Child will start to dislike Mandarin subject and performance can get worse.
Outcome 2b: Child will struggle but manages to catch up with or without some extra help. Child starts to do well in Mandarin.
Outcome 3: Most children who are well-prepared in Mandarin before standard 1 will usually not encounter much trouble understanding what the teacher is saying and is able to learn and keep up in school.
The outcomes we listed down here are from our own experience as parents as well as Mandarin teachers. We have taught students who have 0 background in Chinese, yet do very well in Chinese school because they are well-prepared.
However this takes commitment. Our most successful and fluent kids come for physical classes 5 times a week and they master the language happily. When they come 5 times a week, they will learn the same material 5 times a week but the reinforcement and long-term memory is strong.
This is because learning a language is training muscle memory. Without this constant repetition, you cannot be fluent in a language. Parents please don’t make the mistake by confusing language with Mathematics. Language is like learning a musical instrument. You need daily practise to make it second nature, only then can you be fluent, unlike Maths, where once you understand a concept, you don’t have to repeat it.
Once they start Std 1, they are already fluent with the daily reinforcement in Chinese primary school, they can reduce their lessons or even stop lessons altogether and can still do well in Chinese.