private var arr:Array = new Array(Mc1, Mc2, ..., McN);

var mc1:MovieClip = new arr[0]; // создастся Mc1

removeChild(mc1);

mc1 = null;

var counter:int=-1;

var forward:Array=new Array(clip1_mc,clip2_mc,clip3_mc,clip4_mc,clip5_mc,clip6_mc);

stop();

.....

function goNext(event:MouseEvent) {

        if (video_container_mc.numChildren) {

                video_container_mc.removeChildAt(0);

                //Где-то здесь должен быть null

                <что-то>=null;

        };

        counter=(counter+1)%6;

        var cur_movie:MovieClip=new forward[counter];

        video_container_mc.addChild(cur_movie);

};

.....

next_btn.addEventListener(MouseEvent.CLICK, goNext);

var cur_movie:MovieClip=new forward[counter];

video_container_mc.removeChildAt(0);

var mc:MovieClip = video_container_mc.getChildAt(глубина) as MovieClip;

var counter:int=-1;

var forward:Array=new Array(clip1_mc,clip2_mc,clip3_mc,clip4_mc,clip5_mc,clip6_mc);

stop();

.....

function goNext(event:MouseEvent) {

        if (video_container_mc.numChildren) {

                var mc_for_kill:MovieClip = video_container_mc.getChildAt(0) as MovieClip;

                video_container_mc.removeChildAt(0);

                mc_for_kill=null;

        };

        counter=(counter+1)%6;

        var cur_movie:MovieClip=new forward[counter];

        video_container_mc.addChild(cur_movie);

};

.....

next_btn.addEventListener(MouseEvent.CLICK, goNext);

video_container_mc.removeChildAt(0);

clip1_mc = null;

removeChildAt(0);

video_container_mc.removeChildAt(0);

trace(video_container_mc.numChildren);