Hi,

You will have to set symmetric child nodes' positions around the container's center -

void SetCenter(DiagramNode node, Point center)
{
	var c = node.GetCenter();
	var r = node.Bounds;
	r.X += center.X - c.X;
	r.Y += center.Y - c.Y;
	node.SetBounds(r, false, false);
}

void FlipChildrenX(ContainerNode container)
{
	var bounds = container.Bounds;
	bounds.Y += container.CaptionHeight;
	bounds.Height -=container.CaptionHeight;

	foreach (DiagramNode child in container.SubordinateGroup.AttachedNodes)
	{
		var newChildCenterX = bounds.Right - (child.GetCenter().X - bounds.Left);
		SetCenter(child, new Point(newChildCenterX, child.GetCenter().Y));
	}
} 

A RenderTransform will not affect child nodes because they are not a part of container's visual tree. It's not likely you'd like the result anyway because it also flips the nodes texts making them unreadable. If that's what you are after, you could still apply mirror transform to individual child nodes.

Regards,
Slavcho
Mindfusion