Huh, that's interesting because the multilingual kids I've encountered/heard about do learn how to codeswitch fairly quickly and don't actually use languages interchangeably regardless of context. E.g. one of my Korean-American friends has a cousin who is married to a Latino man; when she spoke Spanish to her cousin's kid (don't know how old but wasn't in school yet), they were very confused and refused to reply in Spanish because they had already internalized codeswitching rules ("father's family speaks Spanish and mother's family speaks Korean") at a really young age. However, that may be because of living in environments where the codeswitching rules are really fixed--i.e. most of the people around them only understand one of the languages/dialects they speak--whereas (I'm guessing that) the switching rules for Singlish and English in Singapore are probably more complex.