Such code cannot be triggered when a node is selected, because that would cause all children to be selected due to recursive behavior. But you could trigger such selection behavior based on a click:
myDiagram.addDiagramListener("ObjectSingleClicked", function(e) {
var part = e.subject.part;
if (part instanceof go.Node) {
var it = part.findLinksOutOf();
while (it.next()) {
var link = it.value;
link.isSelected = true;
var child = link.toNode;
child.isSelected = true;
}
}
});