Set and reference dynamic symbol ID

I need to create a number of symbols dynamically within another symbol and then set the position, height, etc. for each of the newly created symbols.

I've read everything I can find and come up with this code to create a new instance of the symbol "blueRect" within the existing symbol on the stage called "Chart".

for (i=0;i<3;i++) {

  //create new child symbol
  var NewSymbolInstance = sym.createChildSymbol("blueRect", "Chart");

  //create instance name for this new symbol

  NewSymbolInstance.element.attr("id","newIDName"+i);

  //set top position

  sym.getSymbol('#newIDName'+i).css ('top', 250);

}

When I run this code, it creates the first symbol instance and then throws this console message:

SCRIPT5007: Unable to get property 'attr' of undefined or null reference

so it seems  to be choking on the line that creates the instance name.

All thoughts most greatly appreciated!!!!

Carolyn