You didn't really say if your in-laws were planning on hitting the parks with you or not. If not, then anything in Epcot or Disney Studios is out because they'll have to pay for admission.
But if they are doing a day of touring with you or if they have annual passes, definately consider Epcot for dining. Marrakesh is excellent--in my opinion the best eats in all of WDW parks. San Angel Inn is a lovely setting and the kids will probably enjoy it as well. Have you also considered the Teppanyaki Dining Room in the Japan Pavillion? It's like Benihana but with a bit of Disney charm.
And at Disney Studios, don't do the Sci-Fi Dine In Theater (what Maria was trying to tell you about). It's really dark and noisy in there and NOT a good place for snobby in-laws. Instead try Mama Melroses. It's tucked away in a corner of the park (near Muppets 3-D) and the food is really excellent.
Outside the parks, I'd go with 'Ohanas, but tell your in-laws that the food comes "family style" with most of it arriving on huge skewers.
I haven't eaten at the Whispering Canyon, but I heard it was barbecue (and not as good as Tony Romas). The atmosphere and fun and games make it seem kind of nice, but it doesn't sound to me like a place for hard-to-please types.
We do Wolfgang Pucks out our way. There's one in Universal City Walk L.A. as well as Santa Monica Promenade that my kids really enjoy. If the Downtown Disney one is like it, then it will be colorful, kind of fun, with good food. Shouldn't in-laws be impressed that it's Wolfgang Puck?
Lastly, a friend of mine works at House of Blues at Downtown Disney Orlando, and that's always a decent place, especially the Sunday Gospel Brunch.
Don't sweat about your kids. I have first hand recent experience that the restaurant staff expects noisy and wiggly little ones. This is, after all, Walt Disney World and not the Four Seasons.