Several young people walk across the water on a series of stepping stones.
Boys and girls are walking.
The fact that there are several young people does not mean that they are boys and girls.  They could be boys or girls.
exists e1 s1 s2 s3 x1 x2 x3 x4.(n_water(x2) & on(e1,x3) & of(x3,x4) & n_stone(x4) & a_stepping(s3) & Theme(s3,x4) & n_series(x3) & across(e1,x2) & Actor(e1,x1) & v_walk(e1) & n_people(x1) & a_young(s2) & Theme(s2,x1) & a_several(s1) & Theme(s1,x1))
exists e1 x1 x2 x3.(Actor(e1,x1) & v_walk(e1) & subset_of(x3,x1) & n_girl(x3) & subset_of(x2,x1) & n_boy(x2))