Other forms: cacoepies
Cacoepy is the mispronunciation of a word. Your little cousin saying "brefix" instead of "breakfast" is a cute example of cacoepy — but he'll probably grow out of it by the time he's in middle school.
Cacoepy comes from the Greek kakos, meaning "bad," and epos, meaning "word." While it sounds like it might refer to swearing, it specifically means pronouncing a word in a way that's considered incorrect or nonstandard. Because language is fluid, cacoepy can be relative to a regional dialect; however, the term is most often used when a pronunciation ignores the word's pronunciation in any dialect. The opposite of cacoepy is orthoepy, the expected or accepted pronunciations of words.