We had 815am ressies back in December. We got to the park around 720am. (we caught a character breakfast bus around 7am from SSR) You can go through security and get in line for the character breakfasts, but they don't let you into the park until 745am. We waited quite a while and it rained half of that time so I wouldn't show up too early, but the lines were kind of long by the time 745am rolled around. (Others are saying 750, it could have been, but my DH said 745)
You can get through at that time for the 815am ressie. My advice is to check in as soon as you get back to CP. We did and we were seated right when they opened at 805am. I believe we were the 4th family seated. It was great! No wait at all at the buffet! And we were able to get a good bit of our food eatten before the characters started making their way to the tables.
Have a great time with Pooh and Tigger (Eeyore and Piglet too)!