Several children are playing in a fountain.
Children are in the water of a fountain.
Children by itself denotes a more general description of a group of children, which can be rephrased as several children. Since fountains usually contain water, if they are playing in a fountain then they would be in the water of the fountain.
exists e1 s1 x1 x2.(in(e1,x2) & n_fountain(x2) & Actor(e1,x1) & v_play(e1) & n_child(x1) & a_several(s1) & Theme(s1,x1))
exists p1 x1 x2.(n_water(x2) & exists x3.(in(x1,x2) & of(x2,x3) & n_fountain(x3)) & n_child(x1))