Hi Mari-Elena,
You're right you can view the Magic Kingdom fireworks from both the Contemporary and the Polynesian resorts but you can also get a lovely view from the other monorail hotel the Grand Floridian. As your children are so young and you don't want a fancy meal I'd highly recommend
1900 Park Fare, this is a delicious character buffet dinner. The characters are subject to change but the ones that normally make an appearance are Cinderella, Prince Charming, Lady Tremaine and the Step-Sisters. We ate there last year and not only was the food delicious, you must try the strawberry soup, but the characters were great, Anastasia and Drizella had my girls in hysterics!
You can't actually view the fireworks from 1900 Park Fare but once you've finished your meal just take the short walk to the beach for a good view. Another bonus of watching the fireworks from a monorail resort is that you also get to see the Electrical Water Pageant which is a lovely parade of lit floats, it normally goes past the Grand Floridian at 9.15pm.
Hope that helps and you have a great trip,
Jo