This should definitely work:
instance.text="something";
holderMC.addChild(instance);
anArray.push(holderMC);
addChild(holderMC);
trace("instance.text: " + instance.text);
trace("holderMC.instance: " + holderMC.getChildAt(holderMC.numChildren - 1));
trace("holderMC.instance.text : " + (holderMC.getChildAt(holderMC.numChildren - 1)as TextField).text);
You need to cast the Child to TextField, because it getChild... returns a DisplayObject.
This is all assuming that instance is a TextField or another component with a text property.