TikZ anchor a graph at a non-graph-node coordinate

Is there some way to position a graph in TikZ using an anchor that is defined relative to nodes in the graph? For example, I want to position the graph so that the upper left corner of the graph’s bounding box is at a specific point. I see in the manual you can set anchor here and desired at for a particular node in the graph, but I want to anchor elsewhere.

MWE:

documentclass{standalone}

usepackage{tikz}
usetikzlibrary{fit}
usetikzlibrary{shapes.misc}
usetikzlibrary{graphs}
usetikzlibrary{graphdrawing}
usegdlibrary{trees}

begin{document}

begin{tikzpicture}

  node (A) [draw] at (1,1) {A};
  
  graph [tree layout] {
    root -> {
      child 1, child 2
    }
  };
  node [draw, dashed, gray, fit=(root)(child 1)(child 2), inner sep=0] {};
  node [draw, cross out, red] (B) at (child 1.west |- root.north) {};

  node [draw, cross out, green] (C) at (A.south east) {};
  draw [->] (B) -- (C);
  
end{tikzpicture}
  
end{document}