function toggle() {
    var aParents = $A(document.getElementsByClassName('parent'));
    
    aParents.each(function(node) {
        var objToggle = node.getElementsByTagName('a')[0];
        Event.observe(objToggle, 'click', function(event) {
            Event.stop(event);
            var objChild = node.getElementsByClassName('child')[0];
            objChild.toggle(); 
        });
    });
}
Event.observe(window, 'load', toggle);