A boy and a girl dancing with each other.
There are two children in this picture.
If we know that there is a boy and a girl, then we know that there are two children, since boys and girls are both children.
exists e1 s1 x1 x2 x3.(a_topic(s1) & subset_of(x2,s1) & all x4.(n_other(x4) -> with(e1,x4)) & Actor(e1,x3) & v_dance(e1) & (x2 = x3) & n_girl(x2) & subset_of(x1,s1) & n_boy(x1))
exists p1 x1 x2.(n_picture(x2) & in(p1,x2) & exists x3.((x1 = x3) & n_child(x3) & card_eq_2(x3)))