Dear Monorails & Mickey Tales: We have an 8am reservation at Be Our Guest Restaurant, and the Magic Kingdom opens 8am that day too. How early can we get into the park to get all the way back to the restaurant? I was thinking we should get there at least but 7:30, but wanted to hear your thoughts!
Since your reservation is at the same time as the park opening, you'll be let inside the gates at the same time as everyone else. This might be a little before 8am if you're lucky, but it's impossible to know when Disney will do that. There will be a line by 8am, so arriving closer to 7:30am is a safe bet.
If you're able to make a change, consider making a lunch reservation at Be Our Guest instead. Then use that precious first hour to ride attractions that will have much longer waits later in the day.