We tend to like to eat the latest breakfast available...we're not morning eaters normally, but love breakfast food for dinner, so we go late, get more time with the unrushed characters and have always found that the food is replenished appropriately. I've never seen them run out of food just because it's close to time to change meal choices.
We've not done such a late dinner, but I've also read plenty of trip reports saying that the buffets are kept full and nice looking up the end at all the buffet restaurants so that shouldn't be a concern. Honestly, my biggest concern would be a tired child not getting to eat until 8:20 (ok, maybe it would be more my worry about this tired mommy not eating until 8:20). My kids and I tend to get a bit cranky when we don't eat within a couple hours of our normal mealtime....and if we're already tired from a long park day it's only magnified.
However, if you believe your daughter can handle the time....don't worry about the food. Our experience with character meals is that kids rarely eat anyway, they're too excited about who's coming to their table next. But the buffets should be in good shape for those who do want to actually eat, lol.