We really enjoyed the parade, but it does have some points that our Haunted Mansion level scary. There were some bad guys and some skeletons that were rather spooky. It's "Not SO scary" but still kinda scary. I'm not sure how old your little ones are or if they would be affected, but it's something to think about.
We got lucky and caught the fireworks at 'Ohana earlier in the week before our Halloween party, so we skipped the fireworks at the party. We watched from Frontierland, BUT not only is it harder to see the BEST fireworks show, you don't get to enjoy the special themed lighting and halloween music on main street. Frontierland got crowded where we saw the parade there, so I would anticipate Main Street to be crowded. However, I think the atmosphere would be much better and then you're already there to catch the fireworks, so that's what I would choose if we did it again.
Little Ones will probably also delight in meeting Pooh and friends at the thotful spot; they all four wear Halloween Costumes.
Also there were two dance parties last year. Monsters in tomorrowland and villains at Diamond Horseshoe. Those are a big hit with the preschool age.