A children with a helmet riding his bicycle.
A child rides a bike.
Children is meant to be child because A is singular. Bike is a short term for bicycle. Riding is the suffix form of rides.
exists e1 x1 x2 x3 x4.(a_topic(x1) & with(x1,x2) & Theme(e1,x4) & Actor(e1,x3) & v_ride(e1) & n_bicycle(x4) & of(x4,x1) & n_male(x1) & (x2 = x3) & n_helmet(x2) & n_child(x1))
exists e1 x1 x2.(Theme(e1,x2) & Actor(e1,x1) & v_ride(e1) & n_bike(x2) & n_child(x1))