Question regarding controling object from child .as file

If I import pgming from one AS into an overall one like this:

import asFiles.slideshow.*;

public var slideshowAS:Slideshow;

//then later:

slideshowAS = new Slideshow;
mainSprite.addChild(slideshowAS);
//this code works fine in the the as file and loads fine

(mainSprite is a holder in the MainMenu .as file)

how do I control one object that is within that child .as file from the parent .as file?

so for instance if in the slideshowAS there is an item called “nextBtn” and I want to be able to do something with it in the parent main menu .as file.

If you could even direct me on what to search under I that would be very helpful! I tried googling it but did not know how to search for it.

Thanks!