2 books · Ages 7–12
Modern US History Explained is a 2-book series for ages 7–12 by Baby Professor. Each book stands alone — start anywhere.
Start anywhere — each book stands alone.
That said, How The US Became One of the World's Superpowers and US Joins the War: President Wilson's Role in World War 1 are great first picks.
No particular order needed — browse and pick what catches your eye.