That really depend on your kids more then anything else.
My 4 year old doesn't take naps anymore and goes to bed at home at 10:00 at night. However many nights at WDW she falls alseep while in the stroller and misses the night parade. The baby takes a nap during the day, and her bed time at home is 8:00. Some nights if we we still were in the park at 10:00 she was wide away.
If there was a lot of activiy and fun thing they both were pretty happy and stayed up. When we sat down to wait for something like fireworks, they often feel alseep.
Keep in mind if you stay until closing getting home on the bus can be hard with the kids.
However they both ALWAYS feel alseep on the way home. We did have a car this trip (we don't always) and did find it was much eaiser to go home with the car and them both safe asleep in the car seats then us trying to hold them, the double stroller and all our stuff on a very crowded bus (often standing up.)
If we went to one park in the AM and one in the PM we would often take the bus in the morning, but drive in the eveing, excpet to MK. Even then I think the boat to TTC might be better then waiting 30 min while 5 other busses come and go and the line is so long (if you stay to closing.)
We did find that when the kids were alseep when we reached the room it was much easier then trying to get them to fall alseep in the room. They would get second wind then and bounch off the walls!
If you want to view MK fireworks from a distance (good with kids sometimes) the beach from Poly can be a great place.
