This should do the trick:
var nameToSearchFor = "location_mc";
var doc = fl.getDocumentDOM();
var results = fl.findObjectInDocByName(nameToSearchFor, doc);
if (results.length > 0) {
var firstItem = results[0].obj.libraryItem;
var childTimeline = firstItem.timeline;
childTimeline.layers[0].frames[0].elements[0].setTextString('hello world');
}
else {
alert("failed, no objects named " + nameToSearchFor + " found");
}