Where are you staying? Will you have a car?
If the lights go on at 6:00, you'll want to take some time to look at them (.5-1 hour?), then head back to your resort (20-30min?), drop off the kids (10-15min?), change(?), and make your way over to the Grand Floridian, unless you are already staying there. If you don't have a car, you'll either want to take a cab to the GF (10-20min), or transportation to MK, and the boat over to the GF(20-30min).