mitmoo: "The Big Jessie it is true that children can slip between text and grammatical English. To think they can't is like saying if you learn French you'll forget English."
Unfortunately, as I said before, on personal observation (i.e. shoddy anecdotal evidence), that doesn't happen. I even used to believe people could switch, as from French-English actually, as a teenager, because my friends said they could, funnily enough. It was "common sense".
Perhaps it has to do with the brain normalising what we frequently see. For example, "proffession" and "seperate" used to leap out at me when I saw it, but I've seen those mis-spellings so many times, that they no longer do.
So, one can switch between languages, because languages are different enough that you can tell which mode you should be in. But when you are writing a essay under pressure, you can slip into writing "lyk" for "like". Or a mumsnetter perhaps talking about how clever "DD" was this morning, in an email to someone.
That said, the OP says her daughter managed a 7C, which I assume is good, so... Perhaps it's just the circle-of txt-spkers I know that couldn't manage to switch registers.