Dynamic equilibrium occurs when the forces acting on an object are balanced and the object moves at a constant velocity. In the case of the child sliding down the slide, the child is not in dynamic equilibrium while they are sliding because they are accelerating due to gravity and the incline of the slide.
The child reaches a state of dynamic equilibrium when they stop sliding and can continue moving at a constant velocity without accelerating or decelerating, which happens after the slide ends and the child has stopped moving. Therefore, the correct response to when the child is in dynamic equilibrium with the slide is:
"When the slide ends and the child has stopped moving."