How to not select a node in a group

I have some nodes in a group. Now I want that the user can select the group by clicking on the grouptemplate and by clicking on a childnode. I have made a CustomClickSelectingTool todo this (there was an issue here about that long time ago).
If the user double the node I want the node to get selected, and I also want that the node can’t be moved if he is child of a group. I implemented all of that.
But now the problem:
If I hold the mousebutton over the childnode and begin to move (which should move the whole goup) the node get selected.
How can I handle this?

You’ll need a custom DraggingTool (and maybe others) to customize the standard selection behavior of those tools. You’ll note that DraggingTool.standardMouseSelect already is overridden in order to customize the selection behavior.

Here is its regular definition:

    /// <summary>
    /// Don't have the Control modifier unselect an already selected part.
    /// </summary>
    /// <remarks>
    /// This also remembers the selectable <see cref="CurrentPart"/> at the
    /// current mouse point.
    /// </remarks>
    protected override void StandardMouseSelect() {
      Diagram diagram = this.Diagram;
      if (diagram == null || !diagram.AllowSelect) return;
      this.CurrentPart = FindPartAt(diagram.FirstMousePointInModel, true);
      if (this.CurrentPart == null) return;

      // change behavior of StandardMouseSelect because we don't want the Control modifier to unselect
      // an already selected object
      if (!this.CurrentPart.IsSelected) {
        if (!IsControlKeyDown() && !IsShiftKeyDown()) {
        this.CurrentPart.IsSelected = true;

Thank you Walter,
that was easy.