You may have the wrong title- the Fox and the Hound was a Disney, feature-length cartoon pushing inter-canid harmony,. a Fox is not strictly speaking a dog but is in the Canid, as opposed to canine, family.
Well, 1st they need to enter a try out to a movie they would like to be in. O r, they could get into acting lessons if they arent familiar with acting!
Hope I Helped!
A number of talent agencies specialise in child actors, but the first port of call would be a stage or drama club. Some of these in the Los Angeles or New York areas may be associated with talent...