jQuery#children() only returns the immediate children of the element. If you want to dig deeper, you should use jQuery#find()
$('#header2').find('.menu_head'); // returns all divs with the class "menu_head" within the div with id "header2".
$(this).children(); //All Immediate Children
$(this).find('input').show(); // All Children and Sub Children
$('#header2').find('.menu_head'); // returns all divs with the class "menu_head" within the div with id "header2".
$(this).children(); //All Immediate Children
$(this).find('input').show(); // All Children and Sub Children