ok, guys. I found the solution:
I found out that you can have access to movieclip children inside events. so I go for "tick".
var self = this;
this.on("tick", function()
{
self.myChildren_mc.childrensChildren_mc.gotoAndStop(3); // W O R K S !!!
}, null, true); //if true, it's only fired once!
in some cases you have to remove the listener before.