The phrase "Silly rabbit, Trix are for kids" is from a popular ad campaign for an American breakfast cereal called Trix. In the ads, a cartoon rabbit attempts to eat the cereal but the children tell him, "Silly rabbit, Trix are for kids."
The series of television commercials debuted in 1959 and depicted the cartoon rabbit using various disguises in order to obtain the Trix cereal that he loved so much. No matter what he tried, the children would discover he was a rabbit and deny him his fruit-flavored Trix cereal. The cereal has been manufactured by the General Mills company since 1955.