ASP.NET MVC4 Model's Child Collection Drop Down List not binding properly

This syntax works

@Html.DropDownListFor(x => x.ChildCollection[i].ChildID, new SelectList(ViewBag.ChildCollectionSelect as SelectList, "Value", "Text", Model.ChildCollection[i].ChildID))

Define the SelectList as new, then specifically set the selected value from the model.