That must be hard, perhaps allergies? i know I always woke up cranky even though slept well and turns out I was suffering from bad case of allergies. In small children, it's harder to detect that type of pattern, so perhaps bring it up with your DC's ped?
I worried for a bit when my DS went through a week/2 weeks of that; usually wakes up 8am and was often happy initially but cranky or quiet by time we leave his room. As it turns out, he was teething, so that cannot be it for you.
Feel your pain though!
Mummy to DS1-6/11 and DS2-1/14