Help with removing child

[LEFT]Sorry to bring problems but i am trying to learn some as3 and so far i can not resolve this situation ,[/LEFT]

[LEFT]this is the script i am using ,[/LEFT]

[LEFT]Ent.buttonMode = true;
Bio.buttonMode = true;[/LEFT]

[LEFT]ent.addEventListener(MouseEvent.CLICK, goEnt);

function goEnt (e:MouseEvent):void
{
var new Ent:MovieClip = new firstIntro();
this.addChild(newEnt);
newEnt.x = 659;
newEnt.y = 428;
}[/LEFT]

[LEFT]Bio.addEventListener(MouseEvent.CLICK, goBio);
function goBio (e:MouseEvent):void
{
var newBio:MovieClip = new bioIntro();
this.addChild(newBio);
newBio.x = 659;
newBio.y = 428;
}[/LEFT]

the problem is that when i click on first button the Movie Clip does load ok but when i click on second button the first movie cliip is visible , i have try everything i have read here but no luck , i want to learn how to remove first child and allow only the second child to show up , i am looking forward to add more buttons so the remove child should be for multiple buttons , i am learning and so far did purchase a online tutorial but did not help me with this situation , thank you all in advance ,