Kids love surprises, particularly surprises found in candy.
Cracker Jacks have been in business since 1896. Their formula of always including a toy surprise inside the box has made them far more successful than they would have been otherwise. Similarly every Kinder Egg contains a hollow chocolate egg with a yellow plastic container inside containing a toy. Kinder means child in German, and Kinder Eggs use the same formula as Cracker Jacks with the same result.
My boys turned nine this morning. There are 24 kids in each of their school classes. Hence 50 Kinder Eggs were purchased so that each classmate can break open his or her Kinder Egg sometime after lunch to both savour the chocolate and play with the toy. There may even be an extra for each teacher.