Just because a child uses text speak doesn't mean they can't spell in proper writing.
Like Grimma says word play and playing with letters has been around for years.
Text speak is a new form of shorthand in many ways, simple as that.
I can spell but on MN will still use dh, dd, lol, imo, etc.
Text speak started because text messages were a limited set of characters. This has changed a little and increased, although they are still restricted, and if you go over xx characters it costs you another text message to finish your sentence. So it is shorthand to make messaging cheaper. It was also used initially before predicitive text came in, to quicken messaging up.
I chose not to use text speak on my mobile. I have predictive text and text speak would be more of a pain anyway. And I prefer to use punctuation.
But I have known a lot of teenagers use text speak and still be perfectly capable of writing and spelling properly on paper. I have never seen homework or exam answers using text speak for example.
I do think there is a time and place for not using text speak, but I don't see it as a problem. I think some people have a ridiculous level of snobbery over it.