Moving along the curved surface creates a centripedal force towards the center of the curve.
Work Step by Step
As the child and sled come over the top of the hill, they are moving in a curved path, and are therefore accelerating. The net force must be directed downward/inward, toward the center of the curved arc. The 2 forces acting on the child at that instant are gravity (pulling downward) and the normal force from the sled (pushing upward). The net force is downward, so the magnitude of the normal force is less than the child’s weight.