A prize was anything that was awarded to someone who received recognition for some sort of achievement, such as winning a carnival game.
A Kewpie Doll was a type of doll that was given away as a prize for players that were able to beat a certain carnival game. Marvin White gives it a shot but he fails, however a gorilla does it and wins. He then wins the prize.
According to dialogue by Superman in the 1982 Post Cereal Advertisement, the Grand Prize gave nine kids the chance to win a trip to Hollywood and have breakfast with the Super Friends and the supervillain that they created.