Hmmm I don't know! I wouldn't know - though the kids in my friend's experience seem to be unable to tell the difference between Singlish and English at such a young age. This is not necessarily because the languages were very closely related, but because in Singapore, the prevailing idea is that Singlish is a corrupted English, and kids should NOT speak it etc, but yet there would be a lot of the older generations and immigrants who cannot speak Standard Singapore English (very similar to standard RP, but with a Singaporean accent and certain vocab) who can only speak Singlish.

So kids growing up in that sort of environment (with parents/surroundings who can only speak Singlish, not English) would not know it is a different language. You know? Singlish and English are conflated as the same language a lot - if you went down to a neighbourhood place, for example, and met an old shopkeeper who normally spoke Chinese or something, and asked if they could speak English? They'd speak Singlish (if they could).

So I'm not surprised that the kids conflate them.

(also, there's Singaporean Mandarin. I wouldn't be surprised if eventually there would be a dialect for that, too, though because of the growing decrease of bilingualism in Singapore, it probably wouldn't be quite as strong a presence as Singlish would be.)