If you do 8am at CMs, it's pretty unlikely you'll be able to be at the MK at opening.
If we are wanting to get to a park at opening, we do one of two things:
-We either make an 8am breakfast ressie INSIDE that park (like at the Crystal Palace for MK or Donald's for AK)
or
-We head to the park for opening and then have a late (10:30am or so) brunch after we've gotten some of the more important rides out of the way. When we do that, we usually have a cereal bar and fruit or something in the room before we head to the park.