Boolean flag;

protected void TreeView1_TreeNodeCheckChanged(object sender, TreeNodeEventArgs e)
{
    Label1.Text = "check event fired";
    TreeView1.SelectedNode.Checked = true;
    if (flag)
    {
        foreach (TreeNode child in TreeView1.SelectedNode.ChildNodes)
        {
            child.Checked = true;
        }
    }
    else
    {
        TreeView1.SelectedNode.Checked = false;
        foreach (TreeNode child in TreeView1.SelectedNode.ChildNodes)
        {
            child.Checked = false;
        }
    }
}

<script language="javascript" type="text/javascript">
function postbackOnCheck()
{
    var o = window.event.srcElement;
    if (o.tagName == 'INPUT' && o.type == 'checkbox' && o.name != null && o.name.indexOf('CheckBox') > -1)
        { __doPostBack("",""); } }
</script>
<asp:TreeView onclick="postbackOnCheck()" ID="treeContent"
runat="server"> ... And so on ... </TreeView>