Some of my thoughts may depend on what you call early evening but I would think that the Crystal Palace would be a great kick off for the trip! Another option because it is easier to book at dinner than the nearly impossible breakfast would be CRT if your kids are into that.
We have older kids (DD 21 and DS 15) who still insist on eating at the Crystal Palace. We make it there on every trip. It's it's a nice break from the hub bub and who doesn't love Pooh, Tigger, Piglet and Eeyore? However, it's also great as an early breakfast because by the time you're done you're all the way at the end of Main Street and if you time it right you can be at rope drop and get the biggies done early. (Yes, I'm one of those rope drop people)
DH loves it because if we paying OOP we can book the very last lunch time so that we still pay for the lunch buffet but it's more of a dinner for us. DS who is 15 loves it because it's all you can eat and he has reached that "bottomless pit" age. Mom likes it because it is one of the few places you can find veggies in the MK.
Good luck and have a magical trip!