I agree with the post regarding consideration of something besides Ohanah's. We like it there, but it is definitely not the place for picky eaters (because of the set menu & seafodd is a part of the menu). I also like the idea of going to a different resort. Since your cousin & husband do not have passes into the parks. I would suggest the following:
Grand Floridian: 1900 Park Fare (it is a character buffet - good food for a buffet & lots of choices)
Contemporary: Concourse Steakhouse is good. You might want to go to dinner early & then stick around for the fireworks. Go up to California Grill (what I am about to say was true for our visit in March) there is access to the roof top & you can watch MK fireworks from there. Check with someone at Contemporary to see if you are still able to do this.
Boardwalk: ESPN Club is a fun place & I'm quite sure everyone can find something on their menu. There is an arcade right adjacent to the club so the kids can entertain themselves. Spoodles might be a little "adventurous", although we really like the place. Flying Fish is very good.
Yacht/Beach Club: The Yachtsman Steakhouse. This is our favorite restaurant & our kids love it here. It is a little pricey but the food & service are well worth it in my book. there is also an arcade right near the Yachtsman so if the kids finish before the adults & are of an age they can explore on their own, they can occupy themselves while the adults finish dinner & visit.
At either Boardwalk or Yacht/Beach I would plan to eat a little early (or very late) and watch Illuminations (or what you can see from the bridge near both places. You won't be able to see it all, but your cousin & husband can get a little more of the magic while they are there. Have fun!!