A small boy is riding a water slide
The boy is on a waterslide with his brother.
A boy does not have to be small.  Just because a boy is riding a water slide it does not mean he is with his brother.
exists e1 s1 x1 x2 x3.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_slide(x2) & of(x2,x3) & n_water(x3) & n_boy(x1) & a_small(s1) & Theme(s1,x1))
exists p1 x1 x3.(n_brother(x3) & of(x3,x1) & n_male(x1) & n_boy(x1) & exists x2.(on(x1,x2) & with(x2,x3) & n_waterslide(x2)))