To answer your question POLITELY (with no bombing accusations or racist finger pointing), There really is no "supposed to" since everyone has different views. Americans believe in letting children have fun before they have to grow up and take responsibility for themselves. However, most American children are not completely unprepared when it is time to grow up.
Most children learn about the work world when they are young. There are games for children that teach them about different jobs they can have when they grow up. Toy cars and trucks and the likes teach them about driving and about road safety.
Girls DO learn the ways of being a mother. It's called home economics. Boys learn it too, because there is a rise in nontraditional gender roles. More and more, women go to work while the men stay home.
Also, Americans aren't the only ones who do things differently from you. The Europeans, the Mexicans, the Chinese... All of these groups have different ways of raising children. It's not fair to say that Americans are doing things wrong because they do things differently from you.