Welcome Back, Nicole!
This is a tricky one as there are a lot of variables at play. The short answer is Cast Members usually start letting guests through the turnstiles/entry points about 15 minutes before
each park's scheduled opening time. Guests who do get in before the scheduled opening time can usually only go so far as attraction areas are still roped off until the time the park officially opens. The good news is they'll be through the turnstiles, and shops and some Quick-Service dining options will be open. The downside is there will be just a bit more waiting. Pro Tip: Take advantage of the parking lot opening two hours before the park does so your teens can be some of the first in line.
Definitely drop them off at whichever park has Extra Magic Hours that day as that's usually the park with the earliest opening time. A bonus for the rest of this summer is that Disney's Hollywood Studios will host Extra Magic Hours at 7 A.M. EVERY day to celebrate the opening of the
brand new Toy Story Land. I 'Rope Dropped' Animal Kingdom with fellow Disney Parks Moms Panelist Lindsey N in March on an Extra Magic Morning with the hopes of getting to the then brand new land Pandora - The World of Avatar. We were allowed to clear the turnstiles about half an hour before the listed opening time, and at 15 minutes before opening everyone who was there for Pandora was escorted in an orderly fashion to that land. This allowed guests to start queuing up for the attractions as soon as the rope dropped once the park officially opened. I can't one hundred and ten percent say that's how Toy Story Land will operate, but I'm pretty sure it will. So plan to be in line by 6 A.M. at the latest.
I hope this helped, Nicole, and We'll See You Real Soon!
~Kristen