A child in a green and white top sits on a railing.
A child is waiting on some stairs.
A child who is sitting on a railing could be waiting but he may not be.  Likewise, the fact that he is on a railing does not mean that the railing is associated with stairs, the rail could be there for anything.
exists e1 s1 s2 x1 x2 x3.(on(e1,x3) & n_railing(x3) & Actor(e1,x1) & v_sit(e1) & in(x1,x2) & n_top(x2) & a_white(s2) & Theme(s2,x2) & a_green(s1) & Theme(s1,x2) & n_child(x1))
exists e1 x1 x2.(on(e1,x2) & n_stair(x2) & Actor(e1,x1) & v_wait(e1) & n_child(x1))