Jstree - load child nodes after clicking on parent (json)

I want to load the child nodes of a parent node after clicking on the parent node. Because there are many childs this is the only way to keep the performance high.

Now I got this

    <script>

            $(function () {
                    $.support.cors = true;

                    $('#using_json').jstree({
                        "plugins": ["themes", "json_data", "dnd", "wholerow"],

                        'core': {
                            'data': {
                                'url': "http://localhost:56311/ProductRESTService.svc/GetCountryList",
                                'dataType': "json"
                            }


                        }
                    });
 });

this works fine for the first level. Now I want to get the childs from this URL

"http://localhost:56311/ProductRESTService.svc/GetRegionList/{id}",

I do not know how I can achieve this?