The cost to add a fifth day is relatively small, compared to the cost of each of the 4 days already on your hopper. Normally I'd be a bit more on the fence because of the age of your kids and jetlag (though I think I'd still recommend it)...but coming from Hawaii, your kids will probably be wanting to stay up later anyway. So you might as well make the most of it.
Since you are staying on-site, you should get Magic Mornings Tuesday and Thursday. So you'll have PLENTY of time to do Fantasyland and Tomorrowland (if you want) on those two days. You can sleep in a bit on Monday (if you want or need to) to try to ease your transition to the parks. I think I'd plan to see the fireworks and/or Fantasmic! on Sunday, then think about starting the day at DCA on Tuesday...I believe it opens at 10, so that will be early for Hawaii time but is really sleeping in for Anaheim time. Staying at the GCH your family can also easily take naps if needed, though with the relatively short hours, you might be able to do open til close on a few days with just an in-park rest every day.
If you think your kids might be scared, you might just stick with fireworks (you probably know if they like those or not), or show them a video of Fantasmic! to get them used to it first. Or if you want to just be in the park without seeing the shows Sunday evening, I see nothing wrong with that...again, your kids may not feel like it's late at all.
Have a great trip!