We went last year with a new 4 year old and a 8 yr old. We ate dinner at Concourse Steakhouse first. You can enter Mk at 4:00. I would not wear the kids out with another park. Make sure they get a nap, by the time you get them dressed in costume, eat dinner etc. It is a long and short night, if you know what I mean. There is so much to do and so little time to do it in. We did not want to leave early and the kids were getting tired, but they hung in there. If we had wore them out during the day, we would have left by 9:30. If you want to stay busy, maybe go to Downtown Disney for lunch and shop a little. Just a thought.
Tomorrow night is a sell out too. It is going to be very crowded.