I find it very difficult to give you recommendations, without a bit more information. You say it's your first trip and you've only got 3 days. IMO, that leaves a lot of restaurants out because with such a short trip (and never having been there), you won't want to spend your days traveling to resorts etc. for ADR's.
You could have 2 weeks at Disney and really never see it all.
My recommendations are based on restaurants that are in each of the 3 parks (or within close proximity anyway), and I've only included restaurant that require 1 credit for dining. Not staying long also leads me to believe that you won't want to sacrifice a meal to have a signature one (signature requires 2 credits).
The Magic Kingdom day, I'd recommend hoping on over to the Poly and having dinner at the Kona Cafe. Others have liked the Liberty Tree Tavern, but personally, I'm not a fan. You could also head to the Crystal Palace or Chef Mickey's if you're looking for a character meal. The CP and CM also offers character breakfasts if you'd rather use your credits during breakfast. These are buffets and IMO, not the best food in the world, but it depends on what you're looking for.
At Epcot, there is Le Cellier (great steaks), Tepan Edo (just reopened, but is getting great reviews). I've not eaten here yet, but the former restaurant (very similar) was one of our favorites. The Italian restaurant Tutto Italia is just newly opened (haven't eaten here yet either) and is getting lots of great reviews. Many like the Coral Reef, we never did, however.
MGM. IMO, there isn't much worth writing home about here. I love the Brown Derby, but I'm thinking that might be a 2 credit restaurant? I'm not sure. The 50's Prime Time Cafe would probably be a hit with your 10 year old. The problem with the restaurant is, it's so dependent on who your waiter/waitress is regarding how much fun it can be. We got a dud the 1 time we ate here. I also don't enjoy pot roast, meatloaf, and/or fried chicken when I'm on vacation (can cook these better myself), but the food is actually pretty good and I'd imagine it would be lots of fun if you got an interactive wait staff.
There are some other great restaurant in the World, but I chose these only because of your short stay. If it's signature that you're looking for, there are some great restaurant to choose from. If you're willing to sacrifice park time, there are also many other great restaurants to choose from. Some of the restaurants aren't far from the parks, too. It's really tough to answer without knowing more of what you're looking for.