I assume this applies when it comes out of shadows as well?
I'm not sure I understand what you're getting at here. The Hatchlings come into play by coming out of Shadows. Their text, which allows (forces) them to be attached as dupes to their "adult counterparts" only works if the Hatchling is in play. Whether it comes out of Shadows or is put into play by some card effect, they are in play - and once in play, their text activates and applies.
Although we have been interpreting it that the hatchling can attach as it states "if you control X" which we assumed overwrote the standard duplicate restrictions.
You cannot use a character's text while it is in your hand unless it specifically says you can. Period. Well established and documented rule of the game. So since the Hatchlings do not say "from your hand," they do not have anything to do with the standard method for playing dupes. If the adult dragon is in play, you still have to play the hatchling into Shadows, then bring it out of Shadows. Once you do, it will become a dupe, but not before.