Girls sit in costumes.
The girls are performing in a play.
The girls' activity could be something other than performing.  The girls could be at an event other than a play.
exists e1 x1 x2.(in(e1,x2) & n_costume(x2) & Actor(e1,x1) & v_sit(e1) & n_girl(x1))
exists e1 x1 x2.(n_girl(x1) & in(e1,x2) & n_play(x2) & Actor(e1,x1) & v_perform(e1))