Answer
See the graph below.
Work Step by Step
We chose the system to be the child and the sled.
Now we can draw the motion diagram of the system, as we see below.
Noting that when there are no friction forces, the speed of the system is constant and the acceleration of the system is zero as well.
But when the system encounters a rough surface, the speed decreases which means that the system is slowing down at some rate of $\vec a$.
