EmberJS : issue in tree children selection

here is a tree example I got from the Web (sorry, unable to trace that link), which I have put up in Plunkr: http://ift.tt/1NUsHUq

Here is the snippet which is the code to control the checkboxes in the tree:

App.TreeNodeController = Ember.ObjectController.extend({
    isExpanded: false,
    checked: false,
    contentDidChange: function() {

      this.set('checked', App.get('selectedNodes').contains(this.get('content')));
    }.observes('content'),
    toggle: function() {
        this.set('isExpanded', !this.get('isExpanded'));
    },
    addChild: function() {
        var text = window.prompt('Enter text for node:');
        var node = {
           text: text,
           children: []
        };
        this.get('children').pushObject(node);
    },
    checkedDidChange: function() {
        var selectedNodes = App.get('selectedNodes'),
            node = this.get('content');
        if (this.get('checked')) {
            if (!selectedNodes.contains(node)) {
                selectedNodes.pushObject(node);
            }
                this.get('content').children.forEach(function(controller) {
                selectedNodes.pushObject(controller);
                });

        } else {
            selectedNodes.removeObject(node);
            this.get('content').children.forEach(function(controller) {
                selectedNodes.removeObject(controller);
                });
        }

    }.observes('checked')
});

The issue here is : Suppose, I expand a parent node, and then select the parent node checkbox, the children's checkboxes don't get selected.

Can someone please help?