A little boy holding a yellow, plastic shovel, crouches in the sand.
A little boy is holding a shovel and crouches in the sand.
The sentence structure can be either holding or is holding, they mean the same thing.
exists e1 s1 x1 x2 x3 x4 x5 x6 x7.(n_sand(x7) & a_topic(x1) & rel(e1,x4) & rel(x4,x6) & in(x6,x7) & n_crouch(x6) & n_shovel(x4) & of(x4,x5) & n_plastic(x5) & Theme(e1,x3) & Actor(e1,x2) & v_hold(e1) & n_yellow(x3) & (x1 = x2) & n_boy(x1) & a_little(s1) & Theme(s1,x1))
exists e1 s1 x1 x2 x3 x4 x5.(n_sand(x5) & in(e1,x5) & Theme(e1,x2) & Actor(e1,x1) & v_hold(e1) & subset_of(x4,x2) & n_crouch(x4) & subset_of(x3,x2) & n_shovel(x3) & n_boy(x1) & a_little(s1) & Theme(s1,x1))