JQuery Grid-SubGrid for Parent-Child relation

I suggest that you save information from actionSet in an object which you can easy access later. For example you can use userData parameter and fill the userdata part of JSON data inside of beforeProcessing. The create subgrid you can follow the answer or another one.

The demo demonstrate the implementation approach:

It uses the following code

var mainGridPrefix = "s_";

$("#grid").jqGrid({
    url: "Adofo.json",
    datatype: "json",
    colNames: ["First Name", "Last Name"],
    colModel: [
        { name: "givenName" },
        { name: "familyName" }
    ],
    cmTemplate: {width: 100, editable: true, editrules: {required: true},
        editoptions: {size: 10}},
    rowNum: 20,
    rowList: [20, 40, 60],
    pager: "#pager",
    gridview: true,
    caption: "Contacts",
    rownumbers: true,
    autoencode: true,
    height: "100%",
    idPrefix: mainGridPrefix,
    subGrid: true,
    jsonReader: { repeatitems: false },
    beforeProcessing: function (data) {
        var rows = data.rows, l = rows.length, i, item, subgrids = {};
        for (i = 0; i < l; i++) {
            item = rows[i];
            if (item.actionSet) {
                subgrids[item.id] = item.actionSet;
            }
        }
        data.userdata = subgrids;
    },
    subGridRowExpanded: function (subgridDivId, rowId) {
        var $subgrid = $("<table id='" + subgridDivId + "_t'></table>"),
            pureRowId = $.jgrid.stripPref(mainGridPrefix, rowId),
            subgrids = $(this).jqGrid("getGridParam", "userData");

        $subgrid.appendTo("#" + $.jgrid.jqID(subgridDivId));
        $subgrid.jqGrid({
            datatype: "local",
            data: subgrids[pureRowId],
            colNames: ["Due Date", "Note"],
            colModel: [
                { name: "actionDueDate", formatter: "date", sorttype: "date" },
                { name: "actionNote" }
            ],
            sortname: "actionDueDate",
            height: "100%",
            rowNum: 10000,
            autoencode: true,
            autowidth: true,
            jsonReader: { repeatitems: false, id: "actionID" },
            gridview: true,
            idPrefix: rowId + "_"
        });
    }
});

UPDATED: The JSON data used in the demo one can see below. I added id property which is required for jqGrid. I used actionID as the id of the subgrids:

{
    "total": "10",
    "page": "1",
    "records": "78",
    "rows": [
        {
            "id": 10,
            "comment": null,
            "givenName": "Contact A",
            "familyName": "A",
            "actionSet": [
                {
                    "actionID": 1,
                    "actionDueDate": "2012-12-08",
                    "actionNote": "Action 1"
                },
                {
                    "actionID": 2,
                    "actionDueDate": "2012-12-09",
                    "actionNote": "Action 2"
                }
            ]
        },
        {
            "id": 20,
            "comment": null,
            "givenName": "Contact B",
            "familyName": "B",
            "actionSet": [
                {
                    "actionID": 3,
                    "actionDueDate": "2012-12-11",
                    "actionNote": "Action 3"
                },
                {
                    "actionID": 4,
                    "actionDueDate": "2012-12-10",
                    "actionNote": "Action 4"
                }
            ]
        }
    ]
}