Children are performing a routine on stage.
Some kids are on a stage
children are kids, and children are performing a routine on stage can be rephrased as some kids are on a stage.
exists e1 x1 x2 x3.(on(e1,x3) & n_stage(x3) & Theme(e1,x2) & Actor(e1,x1) & v_perform(e1) & n_routine(x2) & n_child(x1))
exists p1 x1.(exists x2.(on(x1,x2) & n_stage(x2)) & n_kid(x1))