A girl rides a unicycle with a child who rides a scooter.
A small human riding
A girl and a child may both not be small, so it can't be inferred that a small human is riding.
exists e2 e1 x1 x2 x3 x4 x5.(with(e1,x3) & Theme(e2,x5) & Actor(e2,x4) & v_ride(e2) & n_scooter(x5) & (x3 = x4) & n_child(x3) & Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_unicycle(x2) & n_girl(x1))
exists s1 s2 x1.(a_topic(x1) & n_riding(x1) & a_human(s2) & Theme(s2,x1) & a_small(s1) & Theme(s1,x1))