If you need to call a function on the main timeline, dispatch an Event from wherever you are, and listen for that event on the main timeline.
That way, you don't have to worry too much about the hierarchical relationship between the child and the main timeline.
Events are a bit tricky to master, but well worth the effort.
Play around with it, and use trace statements to validate your code.
Keep it simple until you understand how it works.
// from a movieclip someplace on the displaylist in your application:
dispatchEvent(new Event("do something", true));
addEventListener("do something", eventHandler);