The U.S. Government Printing Office has a web site  to help kids understand our country. There are different sections for different age groups and grades in school. For K-2, here is how the site explains the role of government (HT: Gregory Adams):
Why do we need a government? Imagine what your school would be like if no one was in charge. Each class would make its own rules. Who gets to use the gym if two classes want to use it at the same time? Who would clean the classrooms? Who decides if you learn about Mars or play kickball? Sounds confusing, right?
This is why schools have people who are in charge, such as the principal, administrators, teachers, and staff. Our nation has people who are in charge and they make up the government.
Hmmm. Not quite what I’d say. Feel free to take a shot at a better explanation in the comments. Remember, this is for kids who are between the ages of five and seven years old.