mucky pup

This phrase is very common & refers to a dirty person. It is an affectionate use and often applied to a child. E.g."Look at the state of your clothes, you mucky pup! You've been rolling around in the dirt with Billy again, haven't you?"
And to a baby:- "Look at the state of your dungarees, you really are a mucky pup!"
How would these sentences go into Greek? ;)

As an affectionate term, I would use "βρωμιαρούλης" and "βρωμυλάκι".

Though politically incorrect, the diminutive of this word may also be used in sense 3b — affectionately, not at all pejoratively, in the case of the diminutive.
http://www.greek-language.gr/greekLang/modern_greek/tools/lexica/search.html?lq=γύφτος&dq=