Answer
.149
Work Step by Step
We need to know the two forces causing the child to go downward. We know that there is the force of gravity pulling the child down the hill: this equals $mgsin\theta$. However, there is also a frictional force acting directly up the hill: this is given by $=F_n\mu$. The normal force is equal to the cosine of theta times the force of gravity, so we find: $F_f=\mu mgcos\theta$. The child is going at a constant speed, so we know that the two forces are equal. Thus:
$\mu mgcos\theta=mgsin\theta\\ \mu=\frac{sin8.5}{cos8.5}=.149$
