To remove all child from a given movie clip simple loop through its all object and remove the type of object you want. Following code does the same for related_mc.sp_related name movie clip:

for (obj in related_mc.sp_related){
// root.mc1[obj] instanceof MovieClip same as typeof(_root.mc1[obj])==”movieclip”
       if (typeof(related_mc.sp_related[obj])==”movieclip”){
         related_mc.sp_related[obj].removeMovieClip();
       }
}

this loop through all child of related_mc.sp_related and remove the instance if its of the type “movieclip”.