They start letting any pre-opening ADR holders into the park at the same time, regardless of what time their ADR is. We had an 8:20 in June and were let in the park at 7:50ish, along with everyone else with a pre-opening ADR. You don't have to wait until 8:20 to get into the park.
I would also pre-order your breakfast. We did so the night before. I only had to wait for one family to pay when I arrived, whereas at the kiosks, the lines to order looked pretty long. We were amongst the first people seated that day and by the time we were done with breakfast the ballroom was pretty much full.
ETA: There was nothing stopping us from wandering Fantasyland freely after our breakfast.