I would suggest Epcot. We love epcot at night, it stays open latest most times of the year and it it beautiful too. After a day in the water parks the kids will be tired. Strolling through WS might be just right. The light up sidewalks and all the street performer are really good for kids too. Have fun.
If everyone is kinda tired, Epcot might just be the place to go. the kids wont be begging to go on lots of rides and the atmosphere is calmer overall. Dont foget to take some time to watch the fountains and the light up sidewalks as you walk out for the night!!!
Does anyone know if EPCOT still has that parade before Illuminations?