code: function swapChildren(child, child2) {
           var index = this.getChildIndex(child);
           var index2 = this.getChildIndex(child2);
  
           if (index !== -1 && index2 !== -1) {
             // swap z index
             var _z = child.pos.z;
             child.pos.z = child2.pos.z;
             child2.pos.z = _z; // swap the positions..
  
             this.children[index] = child2;
             this.children[index2] = child;
           } else {
             throw new Error(child + " Both the supplied childs must be a child of the caller " + this);
           }
         }
docs: Swaps the position (z-index) of 2 children
  @name swapChildren
  @memberOf me.Container.prototype
  @function
  @param {me.Renderable} child
  @param {me.Renderable} child2