We've never had a problem getting to MK early on the buses. We have stayed at all star resorts. For 8AM early dining, we walk to the bus stop at 10 minutes to 7AM. Bus is usually there, or there in less than 10 minutes. Other diners are waiting with you too (usually 6-12 people). You will definitely be surprised to find many other people already at the turnstyles at MK (about 30 or so at this time).
So, if you are walking fast up to the lines you should be in great shape. You will probably be in line about 7:25. The time goes by so fast because, if you are like me, it's so exciting to be there! The CM's first open the turnstyles around 7:50. Then, you move to the right and foward to the arch under the train station. The group of early diners are then held for a few more minutes.
When the CM's let you in, you should be able to walk briskly to the castle for a few pictures before your ADR, with plenty of time to spare. You will be able to get great pictures with no other families in the pics, or possibly only a few families.
Hope this helps. Good Luck.