6X is a size between 6 and 7. (I used to sell children's clothing.) I suppose it's also there to extend "little girl" styles a bit before they graduate into the "big girl" stuff.
Kids sizing is weird--we've got 4T and 5T (and I've seen 6T), and regular 4 and 5 and 6, and in theory, these are supposed to be differences for diapers, but they're really not (depending on the brand). My SD wore T's and nots interchangeably.
ProtoLawyer (the now-actual lawyer, this isn't legal advice, please don't take legal advice from some anonymous yahoo on the Internet)
Spouse (the political geek) * Stepdaughter (the artist) * and introducing...the Baby (um, he's a baby? He likes shiny things).