BROOKHAVEN — Five days into rehearsals for “Singin’ in the Rain Jr.” and teens crowding the historic Haven Theatre stage have already nailed the commandment of one of the classic’s most iconic numbers, “Make ‘Em Laugh.” Director/Music Director Randy Redd chuckled with glee as they danced, romped and performed pratfalls their way through the song. Taylor Newby-Kahre, far right, directs teens in a song and dance number with a funny finish in “Sin…