I Fratelli Pizza gives a free large cheese pizza to the first 50 people who bring in their dog on Smooch-Your-Pooch Wednesdays at all locations in October. More details about Barktoberfest on their Facebook page.
If you and your Cavalier are looking for something else to do after your pizza, The DallasNews.com site has a regular column called Dog About Town about dog friendly businesses and events in DFW. This is a vast and comprehensive list which is worth checking out.