A little boy in an apron helps his mother cook.
they are cooking their pet rock
They indicates more than one. They cannot mean one little boy.
exists e1 s1 x1 x2 x3 x4.(Theme(e1,x3) & Actor(e1,x1) & v_help(e1) & n_cook(x3) & of(x3,x4) & n_mother(x4) & of(x3,x1) & n_male(x1) & in(x1,x2) & n_apron(x2) & n_boy(x1) & a_little(s1) & Theme(s1,x1))
exists e1 x1 x2 x3.(n_rock(x2) & of(x2,x3) & n_pet(x3) & of(x2,x1) & n_thing(x1) & n_thing(x1) & Theme(e1,x2) & Actor(e1,x1) & v_cook(e1))