Spielberg auditioned more than 300 children for the roles in 1982's E.T.: The Extra-Terrestrial. I think it's fairly clear why Henry Thomas was awarded the coveted role of Elliot.
A bit of trivia: listen to the voice of the adult actor who is playing the government agent. Does it sound familiar? It should. It's actor Richard Dreyfuss, a longtime friend of Spielberg's from when they both worked on Jaws together.