What happened to Jeffty as he walked to the movie theatre in the novel, Jeffty Is Five?

Jeffty was standing in line behind two teenagers at the theatre, who were listening to the radio. He asked them if he could borrow the radio to hear a program for a minute while their station was on a commercial break, and the teens had agreed. When he handed the radio back, though, they were unable to pick up the game they had been listening to; the radio was locked in the past. They beat Jeffty viciously and then ran away.