Take the weight of the child (mg), and break it into forces down the plane, and normal to the plane.

Question

down the plane mgsinTheta
normal to plane mgCosTheta

So friction is mu*mgCosTheta. Sliding down, the friction opposes it, so

mgSinTheta-mu*mg*CosTheta=mass*acceleration

From this (frictionless means mu is zero), you can write two equations. Remember that velocity at the bottom is directly porportional to the square root of acceleration. see below.

Vf^2=Vi^2 + 2ad
Vf = sqrt (2ad)

So your basic equation will be..
mgSinTheta-mu*mg*CosTheta=mass*constant*velocity^2

Now make your two equations, and solve for mu.