As long as you have a nice easy day that day, try and get them to have a good nap, or at least some quiet time for a hour or two before you go you should be fine. If your kids like the characters I am sure they will get a kick out of seeing them all dressed up in their Halloween gear too.
I took my DS when he was 4, he was pretty pooped by the time the fireworks came around, but he had a blast. There are loads of characters out and about, including ones you don't get out much. My son was particularly excited to think that he was trick or treating at The Magic Kingdom.