Here we assume that each prize has the same probability to be in a box.
You can model this with a 6 side dice.
Assign each number of the dice to one different prize, then roll the dice and record the number, each roll corresponds to one box of cereal, and the number means that you get that price. When the 6 different numbers in the dice showed up, means that you got all the prices. Now, the total number of rolls that you needed to get the 6 numbers will be equivalent to the number of boxes that you need to buy in order to get the 6 prizes.
