The normal force exerted by the ground applies an average acceleration of
a[ave] = (0 - 4 m/s) / (1 s) = -4 m/s²
(where upward is the negative direction) so the average force felt by the child is
F[ave] = (40 kg) (-4 m/s²) = -160 N
or 160 N directed upward.
Without bending his knees, the average acceleration would instead by
a[ave] = (0 - 4 m/s) / (0.1 s) = -40 m/s²
which makes the average force increase tenfold,
F[ave] = (40 kg) (-40 m/s²) = -1600 N
