Yes, we've had breakfast at 10:30. In fact, that's usually what we do. We hit MK for a couple of hours, then when it starts to get crowded, that's when we leave for breakfast!
We did it in May...MK until about 10.15 I think, the crowds started to roll in, so we left the park and headed to the Kona cafe - I'm sure we didn't get there before atleast 10.30, maybe even later...