Good news, though, since the show isn't closing until August 24th, you still have a ton of time to send last minute fan mail!
Newsies' fan mail address at the Nederlander theater is:
ATTN: [Actor You Are Sending To]
208 West 41st Street
New York City, NY 10036
The show will probably receive a TON of fan mail right before closing, so, to ensure that you get a response, I would highly recommend sending your fan mail to a specific Newsies actor/actress instead of the stage manager. Actors/actresses in this show are usually great about responding, but make sure to hurry and send everything so you get a response before the show closes! Just for your reference, the current cast list can be found at:
Message me if you have any questions about Newsies, or any other show! Good luck with all of your fan mail! :)