As soon as something became slow and boring, they would tune out or change channel. Sesame Street was designed as a series of short sketches. There was a relatively fast pace to the shows and the characters were designed to be attractive and interesting to the children watching. The interest and addictiveness was thought to come from the characters and the pace of sketches in the shows. They weren't simply mindless zombies watching the screen.
Children are sophisticated enough to pay attention when what they are watching is meaningful.
As soon as it becomes difficult or impossible to follow they tune out. The truth is I believe that this trait never leaves us and it is an important result for Interaction Design. For example broadsheet newspapers like The Times, or The Guardian or The Herald Tribune, are aimed at advanced readers with a high school education and probably a college degree. The text is deliberately difficult. For those without much high school these papers are simply too difficult.