I need help please

This is not so hard....

The Executive branch is called the Head Honcho. He....or she in 150 years gets to do what he is told by his donors
Congress are the legitimately elected representatives of Lobbyists, Corporate interests and billionaires. They are called the butt smoochers.
And the Supreme Court are like the nasty 9 dwarves forever trying to repeal women's rights, remove benefits from the needy, and make America's Wealthy Rich Again..... And again, and again

for a third grader I would probably compare it to some of the games that third graders play. That the congress is the ones who are chosen to represent your class against the other classes in the school in an all school competition. And the judicial branch is the annoying group that no one really likes because they read all the game rules to make sure that each team is playing fair.

Well you can find educational websites for kids that explain how the government works. I know elementary school teachers often have their students visit these sites in computer labs. Go to these sites and notice how their information and explanations are simple and easy to understand. Try to copy how these websites are written (without plagiarizing, of course).