How to make a prefab a parent to other prefabs?

I think you need to drag the three waypoints into the Path prefab when you are actually in the prefab editing state. What that means, is this: double click into the Path prefab. Then you are in the prefab editing state. In the Hierarchy, you will see just one gameObject: "Path". Then locate the three waypoint prefabs, and drag them on top of the point gameObject, and now you should have the three waypoints as children of the Point prefab. I hope this helps!