The quick and dirty way is a direct translation from your AS2 code:
But it's better to avoid tight coupling like that. The usual methods of doing this are to either 1) pass in a reference to the function to call or the root movie to avoid traversing the displayList. or 2) dispatch an event from the child that the parent will listen for and act on accordingly.
When you use the displaylist to get objects of a certain type, you are conflating function and display. If you decided to put this movie in another, then root would change and your code would break. You might think that you can use parent instead, but then any change in the display hierarchy like inserting a container would break it. It is more robust, and cleaner, to separate logic from presentation.
addChild(someObj) is exactly equivalent to this.addChild(someObj), and both of those add someObj as a display child to the object which calls that function. In your case, they add to the document instance, which is the same as root.