Put movie behind existing elements, how?

Question

I am placing a movie clip in the swf by AS.   In the resulting swf, It's hiding the buttons that are already on the stage.  How do I get that movie clip to be behind other elements on the stage?
Here's the background and details:  In Flash CS4, I have existing "Forward" and "Back" buttons on the stage, frame1 of the topmost layer Artwork, placed manually, nothing else on the stage.  I made a movie clip and it exists in the library but is not on the stage, it's called up via AS, which is in frame 1, actions layer (the layer below the artwork). My AS3 looks like this:
forward1_btn.addEventListener(MouseEvent.CLICK,gotoNextFrame);
back1_btn.addEventListener(MouseEvent.CLICK,gotoPrevFrame);
var mainmovie:mainMovie_mc=new mainMovie_mc();
addChild(mainmovie);
function gotoNextFrame(event:MouseEvent):void
mainmovie.nextFrame();
function gotoPrevFrame(event:MouseEvent):void
mainmovie.prevFrame();
Works great, very effecient, thanks to a teacher, except the buttons are hidden underneath the movie clip.  I can prove that by editing the clip and moving the artwork in the movie clip off to one side, and then re-run the preview and the buttons are visible and work, but I want the movie clip elements in their original location, just want it to go underneath, back to the background.
I've already tried obvious methods like placing the layer that has the buttons as the topmost layer, and right clicking on the buttons and going to "arrange, bring to front."  Doesn't fix it.
If I had to guess, I need a property for the placeChild to make it "send to the back".  How do I do that?
Please give me the exact code and location, I'm just a newbie to AS.  Thanks.
Dave N

Answer 1

That was exactly the ticket, I just changed my addChild line to:
addChildAt(mainmovie,0);
Worked perfectly!  Thanks.
Dave