Hi April,
Yes, with an early morning Advanced Dining Reservation at Cinderella's Royal Table, you'll be able to gain early access to the Magic Kingdom. When you approach the turnstiles at the entrance to the Park, head to the far right. You will find one specifically marked for guests with breakfast reservations. Be sure to have your confirmation number with you.
While Main Street will be
much less crowded than usual, there will be other guests in the Park with you. Crystal Palace also has breakfast early in the morning and those guests will enter early as well.
If I were you, I'd shoot to be at the turnstile at 7:30am. This will give you ample time for photos in front of the castle (with no one in the back ground). Early morning is my favorite time to visit the Parks. In fact, we have a tradition of eating at Crystal Palace first thing in the morning on each trip. You may want to try out
Donald's Safari Breakfast at the Tusker House, as entering the Animal Kingdom early in the morning is especially fun.
Hope this helps and have a wonderful vacation,
Kaylene