1. override
bool hitTestChildren(HitTestResult result, { Point position })

Override this method to check whether any children are located at the given position.

Typically children should be hit-tested in reverse paint order so that hit tests at locations where children overlap hit the child that is visually "on top" (i.e., paints later).

Used by hitTest. If you override hitTest and do not call this function, then you don't need to implement this function.

Source 

@override
bool hitTestChildren(HitTestResult result, { Point position }) {
  final List<RenderBox> children = getChildrenAsList();
  for (int i = _lastPaintOrder.length - 1; i >= 0; --i) {
    final int childIndex = _lastPaintOrder[i];
    if (childIndex >= children.length)
      continue;
    final RenderBox child = children[childIndex];
    final FlowParentData childParentData = child.parentData;
    final Matrix4 transform = childParentData._transform;
    if (transform == null)
      continue;
    final Matrix4 inverse = new Matrix4.zero();
    final double determinate = inverse.copyInverse(transform);
    if (determinate == 0.0) {
      // We cannot invert the transform. That means the child doesn't appear
      // on screen and cannot be hit.
      continue;
    }
    final Point childPosition = MatrixUtils.transformPoint(inverse, position);
    if (child.hitTest(result, position: childPosition))
      return true;
  }
  return false;
}