This can be done using the plain JavaScript and does not need to be passed to the server. You can just write a JS function to reflect the changes from the child to the parent in the following manner

Code:
<script language="javascript">
function reflect()
{
	parent.opener.window.document.<FormName>.<FieldName>.value = document.<ChildFormName>.<Field>.value;
	return true;
}
</script>