Paul Simon has rounded up a few friends for a Dec. 13 Madison Square Garden concert to benefit New York's homeless children. Among those scheduled to appear: Grace Jones, Debbie Harry, Ruben Blades, James Taylor, Lou Reed, Dion, Laurie Anderson and the Paul Shaffer band. Simon, Reed, Blades and Taylor will take on a new collective identity as the "Belmonts" to provide backup for Dion when the '50s star sings "Teen-ager in Love." Proceeds will go to the Medical Mobile Van, which provides health care for the children. Simon has already raised more than $300,000 for the project.