Well, your two best choice's for eating with young children is 50's Primetime or Sci-FI.
I would recommend Sci-Fi because it is one of my favorite Park Restaurants and you really have the best atmosphere in WDW.
You get to watch old time "SciFi" movies in old cars. The food for me was always very good. Not only burgers and fries. I usually get the Reuben sandwich for lunch and love it.
All the times I've been there the servers were very friendly and the service was quick. You can't miss this place on a MGM day.