Hi Jennifer! Your Disney countdown has begun with a summer vacation arriving sooner then you think. I am sure the family is getting very excited!
Park Hours for the Theme Parks is something that is difficult to predict. Disney sets general hours and then will adjust for crowds. You are correct that right now they are stating 9pm closing for the Magic Kingdom park through the summer. We all know that this will change especially when they have closing at 9pm even on the 4th of July.
Since the Animal Kingdom is now open for night time hours Disney is going to be looking carefully at how the crowds disperse for evening hours to all four parks. With the new Pandora attractions opening in the Animal Kingdom I am sure some will be flowing over there to enjoy the new experiences.
Disney does not have a set schedule when and if they change park hours. I have actually been in the Magic Kingdom in the past when they announced to the guests that they would be keeping the park open until midnight. I know they don't prefer to do this at the last minute but if the guest count deems it necessary they will. This is the one park that the hours fluctuate greatly.
Best advice at this time is to keep checking back every week or so as summer gets closer. I am confident you will see the Magic Kingdom park hours change. After all, who wants to see fireworks when it is still light out at 9pm?