First off, to clarify some confusion about the different seating areas:
The Ballroom is the largest of the three rooms and looks exactly like the room where Belle and the Beast dance in the movie. To me, it is the most magical/movie-like of the three rooms and even though it's nosier, I prefer to sit there. The "windows" have a beautiful snow effect that's just mesmerizing!
The West Wing is the smallest of the three. It's very dark and intimate. It features the enchanted rose, plus a really neat portait of the prince. Listen to the thunder - when it gets louder, keep an eye on the painting and watch for a cool effect!
The Rose Gallery features a large music box of Belle and the Beast in the center of the room. It is larger than the West Wing but quite a bit smaller than the Ballroom, and I think it's a bit more brightly lit. It certainly feels airier. If sitting in this room interests you, be sure to go for lunch as it is usually closed during dinner.
As PPs have mentioned, you can take your chances on snagging a walkup. From what I have read, people have had the most success with this right around the time dinner opens. If you'd rather just stick with lunch, another way to go besides sending someone to get in line at 11 is to wait until 1:30-1:45 or so. My parents and I did this a couple of weeks ago and I believe it took us about 20 minutes to get to the front of the line. Going at this time is especially nice because I believe they close off the line at about 2:15. This means that the restaurant only gets emptier and quieter the longer you're there. They didn't rush us out or anything, so we had as much time as we wanted to explore a less-crowded restaurant.
I will also add that if your son would like to meet the Beast, this can only be done at dinner. It's not a character meal as he doesn't visit your table, but he does take a (loudly announced) walk through the restaurant every 30 minutes or so, then meets for photos in his study. This is the area at the front where you place your order at lunch. Don't think he does autographs.
I hope something works out for you and your family to experience BOG! Have a magical trip