Mootools get the child-index of an element from its parent

The type (Elements) returned by getChildren contains Array methods, including indexOf. MooTools will provide an implementation of that method if it does not exist for the browser. With that in mind, you could write:

$('Record_List').getChildren('div.Row').indexOf(rowElem);

Updated example: http://jsfiddle.net/andrewwhitaker/uJarB/