I think it depends a lot on your family and your plan for the day. Our kids are 7, 7, 3 & 2 so we usually plan at least 2 days for MK so our two toddlers can ride everything during the day, then the older kids and I ride more things and repeat rides at night. Here's what we do:
MK1 morning - CP or CRT, Fantasyland, lunch, Tomorrowland (minus Space Mountain), return to room around 2pm
MK1 evening with 7yos - QS dinner, Space Mountain/Buzz, Liberty, Frontierland, Adventureland, parade, fireworks
MK2 morning - CP or CRT, Fantasyland, Liberty, lunch, Frontierland, Adventureland, return to room around 2pm
MK2 evening with 7yos - QS dinner, kids pick (usually mountains, some Fantasyland, Haunted Mansion, POTC, Jungle Cruise, Buzz), fireworks