Greywolf -
FWIW, Sanrio would crucify you in court on a copyright claim. Parody is a defense to copyright infringement, but where the defendant is using it to derive monetary gain, the courts tend to see it more of a derivative work than a parody. As my copyright prof once said, "its really all about the money". Two classic examples...
Luther Campbell's version of Roy Orbison's song "Pretty Woman" (Campbell v. Acuff-Rose). Campbell wrote and recorded a rap version of Pretty Woman. Different lyrics (about the usual drug dealing, 'ho fucking, inner-city jamboree), but the same basic melody. Campbell claimed the parody defense, but the Supremes (and I don't mean Diana Ross et al), shot him down because basically, it wasn't done so much to make fun of the original version as to use the recognition of Roy Orbison's song to make money for himself (Campbell).
Naked Mickey? (Disney v. Air Pirates). Air Pirates did a comic book 'parody' of some Disney characters engaged in various sexual acts and doing drugs. Again, Air Pirates used the parody defense, but the court, as in Campbell) shot them down. Actually, IIRC Air Pirates was years before Campbell, but you get the idea...
The parody defense is very limited in scope, and generally the more money you make from a parody, the less likely it is to be seen by the courts as a valid parody and non-infringing use.
As for trademark protection, you'd need to file with the USPTO, but because of the similarity to Sanrio's existing characters (which I'm sure are already trademarked worldwide), the USPTO would deny your application. Sanrio, OTOH would have claims against you in trademark for unfair competition, dilution, trademark infringement, disparagement, etc. And unlike copyright where the creator owns the copyright, trademarks are granted by the gov't as an indicator of source. You can own a copyright, but you cannot truly own a trademark - you merely control it for a while. That is, once you've got a copyright, its yours for the statutory duration. Trademarks however require the 'owner' to use them in trade AND police and protect them. If you don't, you risk losing your mark. Thus, Sanrio could let you fly on the copyright claims if they didn't think you were a big enough fish to bother frying, but to protect the viability of their trademarks, they'd need to take action if they became aware of your infringement.
Basically, the courts would very likely see your Kalashnikitty idea as a derivative work of Sanrio's characters. If you had a license to make such a derivative work from Sanrio, you could proceed against AK47.net, but without such license, you are tempting copyright and trademark action against yourself and AK47.net by Sanrio.
Cool idea though - I do like the shirts. Now if you want to create trouble for the guys ripping off Kalashnikitty, send an anonymous email to Sanrio's legal department with a link to the site of such sale.