A child is wading through water towards the beach.
A little boy and his sister are playing at the beach.
The first sentence mentions one child, there's no way to infer that it is a boy and his sister. Also, they could just be wading through water and not necessarily playing.
exists e1 x1 x2 x3.(n_beach(x3) & towards(e1,x3) & through(e1,x2) & n_water(x2) & Actor(e1,x1) & v_wade(e1) & n_child(x1))
exists e1 s1 x1 x2 x3 x4.(n_beach(x4) & at(e1,x4) & Actor(e1,x1) & v_play(e1) & subset_of(x3,x1) & n_sister(x3) & of(x3,x2) & n_male(x2) & subset_of(x2,x1) & n_boy(x2) & a_little(s1) & Theme(s1,x2))