Two children walking on a platform.
Children on a platform.
Children on a platform  can only be when they are walking.
exists e1 x1 x2 x3.(a_topic(x1) & on(e1,x3) & n_platform(x3) & Actor(e1,x2) & v_walk(e1) & (x1 = x2) & n_child(x1) & card_eq_2(x1))
exists x1 x2.(a_topic(x1) & on(x1,x2) & n_platform(x2) & n_child(x1))