When the 300-plus kids in the Toronto Children’s Chorus take the stage at Roy Thomson Hall, they are accompanied by the likes of tenor Ben Heppner and conductors Sir Andrew Davis, Sir David Willcocks and Sir Simon Rattle. Formed in 1978 by visionary teacher Jean Ashworth Bartle, the world-renowned choral ensemble grew out of a one-off performing and recording gig with the TSO. Now Elise Bradley and her singers tour the globe and record works by such names as Mahler, Bach and Britten. Children six to 16 years old can audition every spring for one of the half-dozen choirs.