Kids are playing in the sand.
Kids are making a sandcastle on the beach.
The kids could be doing other things in the sand other than building a sandcastle (digging holes, finding shells, etc.). They also could be in a sandbox rather than at a beach.
exists e1 x1 x2.(n_sand(x2) & in(e1,x2) & Actor(e1,x1) & v_play(e1) & n_kid(x1))
exists e1 x1 x2 x3.(n_beach(x3) & Theme(e1,x2) & Actor(e1,x1) & v_make(e1) & on(x2,x3) & n_sandcastle(x2) & n_kid(x1))