Haven't juvenile t-rexes been found and is Nanotyrannus a valid genus?

There are certainly specimens of young animals that are identified as belonging to Tyrannosaurus with some confidence. There are other specimens that have been identified as 'merely' young Tyrannosaurus or as a new small genus called Nanontyrannus. However most researchers I know are happy with the idea that this genus is not valid and all these things are younger specimens of Tyrannosaurus.