The cast of “Billy Elliot” celebrated their opening night performance with an after-party at Fleming’s Prime Steakhouse & Wine Bar. All four of the young actors who play Billy in the musical made the scene, as did Quincy’s own Rich Hebert, who plays Billy’s dad in the show. During the eight performances of “Billy Elliot” at the Opera House, each of the child actors — including Noah Parets of Sharon — will play Billy two times.