To the OP: Just remember that the nonstop flight out of LAX to MCO is slightly less than 5 hours long. Unless your kids are veteran travellers, I would expect the excitement of being on a plane will keep them from falling asleep immediately, so figure they might get 4 hours of sleep. How do you think they will do with napping both before and after the flight?
As I previously posted, my expereince on a red eye was with a toddler who was used to napping just about anywhere (she fell asleep with her head on the dinner table one night at WDW), so she easily made up for the lack of sleep she had suffered on the flight.
You might want to ask in another post for those with experiences with kids 8 and 12 on a red eye.
Also, there is no guarantee that you will be able to check in before 8 am to any hotel when you arrive unless you book the room for the night before (and tell them that is what you are doing so they don't mark you down as a no-show). That said, after my one experience with a red-eye to MCO, I was able to check in to the BW at about 7:30 AM and even though the hotel was very full, they had a room ready for me.
Best of luck -- Suzanne