I did a very similar party for my daughter's birthday this year. Pepridge farm goldfish mixed into chex mix looks very much like cat food. We also made ear headbands, colars, and "doggie" bags to take home. I used liquid eyeliner to make wiskers on the kids faces. They went nuts over the whole theme!
I forgot to say how I made the costume! So here it is. I took a black sock and cut the foot off. Then I cut arm holes to make a doggie sweater. I found thi little man at the dollar store. He was perfect. His arms and legs were wired so I was able to position them how I wanted. You can't see it in the picture but, I sewed a piece of cream colored yarn around the top of the sock and to the toy hands so it looked like he was holding on. I sewed the legs of the rider to the sock/sweater and away we went! Cooper is used to wearing sweaters so he didn't even notice the little man. When he would play the man would bounce around and look funny.
Cooper wanted to be something very masculine for halloween since he is so small,( 6 lbs.) He thought of a cop, cowboy, fireman, or wrestler. After a long talk this is what we came up with. He was happy. He realy got into his roll as bucking bull.