Young kids are on a small train ride.
The children are siblings.
Kids sharing a small train ride don't have to be siblings.
exists p1 s1 x1.(exists s2 x2 x3.(on(x1,x2) & n_ride(x2) & of(x2,x3) & n_train(x3) & a_small(s2) & Theme(s2,x2)) & n_kid(x1) & a_young(s1) & Theme(s1,x1))
exists p1 x1.(n_child(x1) & exists x2.((x1 = x2) & n_sibling(x2)))