Controlling movieclip from main timeline & Child

Question

Hi,
This code is currently contained within my "mc_1" movieclip. I want to be able to have this code on my main timeline, however no matter what I try I constantly get errors, multiple different versions. I've tried adding mc_1 infront of everything, but that just gives an output error about a Sprite.
Also, I'm currently using visible to control whether the back image to the textbox should show - What I'd really like to do is control it using a child, but they're movieclips not buttons, so when I try to addChild within the function it errors saying I'm trying to control a movieclip within a simplebutton function.
import flash.events.MouseEvent;
stop();
mc_textbox.visible = false;
for(var i:uint=1; i<8; i++){
    this["line"+String(i)].addEventListener(MouseEvent.ROLL_OVER, line_in, false, 0, true);
    this["line"+String(i)].addEventListener(MouseEvent.ROLL_OUT, line_out, false, 0, true);
function line_in(e:MouseEvent):void // Defineing the function as line_in
    var lineTarget:SimpleButton = SimpleButton(e.currentTarget); // The variable is lineTarget
    switch(lineTarget) // Each case should swap the lineTarget
        case line1: // line 1 is the first button
        line_output.text = "1"; // This text should display
        mc_textbox.visible = true; // The textbox should display
        break; // If this is not the case, break and check next
        case line2:
        line_output.text = "2";
        mc_textbox.visible = true;
        break;
        case line3:
        line_output.text = "3";
        mc_textbox.visible = true;
        break;
        case line4:
        line_output.text = "4";
        mc_textbox.visible = true;
        break;
/* This case has much more text than the rest, as such the textbox is enlarged for this case only
and changed in position. The line_output dynamic text field is also moved on the Y axis for this.
This is reset to default using the line_out function.
        case line5:
        line_output.text = "55";
        line_output.y = 427.65;
        mc_textbox.visible = true;
        mc_textbox.height = 100;
        mc_textbox.y = 417;
        break;
        case line6:
        line_output.text = "6";
        mc_textbox.visible = true;
        break;
        case line7:
        line_output.text = "7";
        mc_textbox.visible = true;
        break;
        default: // If none of the cases are true, this should display
        line_output.text = ""; // line_output should be blank
        mc_textbox.visible = false; // mc_textbox should be invisible by default
function line_out(e:MouseEvent):void
        line_output.text = "";
        line_output.y = 474.65;
        mc_textbox.visible = false;
        mc_textbox.height = 61.95;
        mc_textbox.y = 454;

Answer 1

I've resolved the timeline issue. Now my only issue is the Child not working.
What I'm trying to do:
addChild(mc_1.mc_textbox) to appear upon rollover of a button. On rolloff, for the child to be removed.
Add mc_textbox inside mc_1 on rollover, at a specific XY and height. On rolloff, for the child to be removed.
I have also been told that I should not have a dynamic textbox on the scene and should call that in using AS3 - If you have time, could you inform me how this would be done?
Thanks alot for the help,
ERROR:
TypeError: Error #2007: Parameter child must be non-null.
    at flash.display::DisplayObjectContainer/addChild()
    at Main_new_fla::MainTimeline/line_in()
TypeError: Error #2007: Parameter child must be non-null.
    at flash.display::DisplayObjectContainer/removeChild()
    at Main_new_fla::MainTimeline/line_out()
import flash.events.MouseEvent;
stop();
for(var i:uint=1; i<8; i++){
    this.mc_1["line"+String(i)].addEventListener(MouseEvent.ROLL_OVER, line_in, false, 0, true);
    this.mc_1["line"+String(i)].addEventListener(MouseEvent.ROLL_OUT, line_out, false, 0, true);
function line_in(e:MouseEvent):void // Defineing the function as line_in
    var lineTarget:SimpleButton = SimpleButton(e.currentTarget); // The variable is lineTarget
    switch(lineTarget) // Each case should swap the lineTarget
        case mc_1.line1: // line 1 is the first button
        mc_1.line_output.text = "1"; // This text should display
        addChild(mc_1.mc_textbox); // The textbox should display
        break; // If this is not the case, break and check next
        case mc_1.line2:
        mc_1.line_output.text = "2";
        addChild(mc_1.mc_textbox);
        break;
        case mc_1.line3:
        mc_1.line_output.text = "3";
        addChild(mc_1.mc_textbox);
        break;
        case mc_1.line4:
        mc_1.line_output.text = "4";
        addChild(mc_1.mc_textbox);
        break;
/* This case has much more text than the rest, as such the textbox is enlarged for this case only
and changed in position. The line_output dynamic text field is also moved on the Y axis for this.
This is reset to default using the line_out function.
        case mc_1.line5:
        mc_1.line_output.text = "55";
        mc_1.line_output.y = 427.65;
        addChild(mc_1.mc_textbox);
        mc_1.mc_textbox.height = 100;
        mc_1.mc_textbox.y = 417;
        break;
        case mc_1.line6:
        mc_1.line_output.text = "6";
        addChild(mc_1.mc_textbox);
        break;
        case mc_1.line7:
        mc_1.line_output.text = "7";
        addChild(mc_1.mc_textbox);
        break;
        default: // If none of the cases are true, this should display
        mc_1.line_output.text = ""; // line_output should be blank
        addChild(mc_1.mc_textbox); // mc_textbox should be invisible by default
// Removes any text that has been displayed from the Dynamic text box, also resetting the sizes to default.
function line_out(e:MouseEvent):void
        mc_1.line_output.text = "";
        mc_1.line_output.y = 474.65;
        removeChild(mc_1.mc_textbox)
        mc_1.mc_textbox.height = 61.95;
        mc_1.mc_textbox.y = 454;