You can define a function 'reloadCombogrid' in the 'CHILD.HTML' page. Find the <iframe> and get the 'contentWindow', call any functions defined in it.
var dechild = $('#tt').tabs('getTab', 'CHILDMENU');
if (dechild != null){
var cw = dechild.find('iframe')[0].contentWindow;
cw.reloadCombogrid();
}