controlling the expandability of list using a checkbox in parent row

Question

I am trying to create an expandable list that has a textview and a checkbox in parent row and has a single child row with multiple child items in it. I want to control the expansion and contraction of the parent row with a checkbox being checked and unchecked respectively, which i am failing to achieve. I am using 3 XMLs: activity_main.xml that has an expandable list; list_group.xml (parent row) that has a textview and a checkbox; list_item.xml (child row) that has two textviews. Here is my code:

public class ExpandableListAdapter extends BaseExpandableListAdapter {
private Context _context;
private List<String> _listDataHeader; // header titles
// child data in format of header title, child title
private HashMap<String, List<String>> _listDataChild;
CheckBox cb_group;

public ExpandableListAdapter(Context context, List<String> listDataHeader,
        HashMap<String, List<String>> listChildData) {
    this._context = context;
    this._listDataHeader = listDataHeader;
    this._listDataChild = listChildData;
}

@Override
public Object getChild(int groupPosition, int childPosititon) {
    return this._listDataChild.get(this._listDataHeader.get(groupPosition))
            .get(childPosititon);
}

@Override
public long getChildId(int groupPosition, int childPosition) {
    return childPosition;
}

@Override
public View getChildView(int groupPosition, final int childPosition,
        boolean isLastChild, View convertView, ViewGroup parent) {

    final String childText = (String) getChild(groupPosition, childPosition);


    if (convertView == null) {
        LayoutInflater infalInflater = (LayoutInflater) this._context
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        convertView = infalInflater.inflate(R.layout.list_item, null);
    //    cb_group.setChecked(true);
    }

    TextView txtListChild = (TextView) convertView
            .findViewById(R.id.lblListItem);

    txtListChild.setText(childText);
    return convertView;
}

@Override
public int getChildrenCount(int groupPosition) {
    return this._listDataChild.get(this._listDataHeader.get(groupPosition))
            .size();
}

@Override
public Object getGroup(int groupPosition) {
    return this._listDataHeader.get(groupPosition);
}

@Override
public int getGroupCount() {
    return this._listDataHeader.size();
}

@Override
public long getGroupId(int groupPosition) {
    return groupPosition;
}

@Override
public View getGroupView(int groupPosition, boolean isExpanded,
        View convertView, ViewGroup parent) {
    String headerTitle = (String) getGroup(groupPosition);
    if (convertView == null) {
        LayoutInflater infalInflater = (LayoutInflater) this._context
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        convertView = infalInflater.inflate(R.layout.list_group, null);
    }

    TextView lblListHeader = (TextView) convertView
            .findViewById(R.id.lblListHeader);
    lblListHeader.setTypeface(null, Typeface.BOLD);
    lblListHeader.setText(headerTitle);

    cb_group = (CheckBox) convertView.findViewById(R.id.cb_group);
    cb_group.setChecked(isExpanded);


    return convertView;
}


@Override
public boolean hasStableIds() {
    return false;
}

@Override
public boolean isChildSelectable(int groupPosition, int childPosition) {
    return true;
}

The main activity code is:

public class MainActivity extends Activity {

ExpandableListAdapter listAdapter;
ExpandableListView expListView;
List<String> listDataHeader;
HashMap<String, List<String>> listDataChild;
CheckBox cb_group;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // get the listview
    expListView = (ExpandableListView) findViewById(R.id.lvExp);

    // preparing list data
    prepareListData();

    listAdapter = new ExpandableListAdapter(this, listDataHeader, listDataChild);

    // setting list adapter
    expListView.setAdapter(listAdapter);

//   cb_group=(CheckBox) findViewById(R.layout.cb_group);
}

/*
 * Preparing the list data
 */

private void prepareListData() {
    listDataHeader = new ArrayList<String>();
    listDataChild = new HashMap<String, List<String>>();

    // Adding child data
    listDataHeader.add("Blackberry Curve");
    listDataHeader.add("Samsung");
    listDataHeader.add("IOS");

    // Adding child data
    List<String> one = new ArrayList<String>();
    one.add("Curve 3110");

    List<String> two = new ArrayList<String>();
    two.add("Galaxy Ace");


    List<String> three = new ArrayList<String>();
    three.add("Iphone 5");


    listDataChild.put(listDataHeader.get(0), one); // Header, Child data
    listDataChild.put(listDataHeader.get(1), two);
    listDataChild.put(listDataHeader.get(2), three);
}