Are you using transparency? Or does
return OpenGL ?
If so then note that OpenGL does not pay attention to the child order: it works in (projected) Z order. It also has rules about what happens if there is a line and a surface in the same plane: the order is determined by the graphic type. I never bothered to remember which order OpenGL defines in this case, as I have encountered graphics driver that get the order exactly backwards.
In the case of OpenGL, you need to make sure that the item that you want to be on "top" has a higher (projected) Z coordinate, closer to the front from the perspective of the viewer. That can involve using scatter3() instead of scatter() and specifying a constant Z coordinate above or below the implied Z=0 of other objects.