The ball describes a parabola, as you can see in the attached picture. So, the point where the ball strike the ground is the point where the parabola meets the x axis. In fact, the x-axis is the set of points where y=0, which means that the ball has height 0 or -again- it hits the ground.
So, we have to set y=0 in our equation and look for the positive solution. We have
And the positive solution is
So that's the distance from the child where the ball strikes the ground.
