We have tried every version of attempting to keep our child awake for fireworks.
Even at 6 y.o., we still have not found a winning formula.
Her ability to stay awake is dependent on luck.
Best success we ever had was accomplishing a LONG afternoon nap after being out from early morning.
Frankly, I am often amazed at the number of small children awake inside MK past 9pm. Some children are troopers, others are simply cranky (or worse).
My advice: if you go to (near) MK, be prepared to let child sleep in stroller. And yes, having a stroller in MK with the crowd is not fun for parents, but it's Disney.