Slang existe since there are men over the world. If you read history you'll notice it for sure! Did you read any letter written by French Revolution protagonist? (Danton, Saint-Just, Hébert)...Lot of French slang there! And they wrote in slang even in some important newspapers, LOL. :DAnd what I found hilarious is that this slang is used even today in French language. Some of the words are now out of use, but most of them are popular nowadays.
So, is not so rare that OTMAA used lots of slang in their letters and other written stuff. If grown ups would use it, why not children? Children will be children, after all.
RealAnastasia