1. override
void addChildAt(Bitmap child, int index)

Source 

@override
void addChildAt(Bitmap child, int index) {
  if (index < 0 || index > _children.length) {
    throw new ArgumentError("The supplied index is out of bounds.");
  } else if (child.parent == this) {
    _addLocalChildAt(child, index);
  } else {
    child.removeFromParent();
    _children.insert(index, child);
    child._parent = this;
  }
}