We take our 3 year old DS out to nice and regular restaurants regularly at home and we did the same when we took him to WDW when he was 19 months and 2.5 years. As everyone is saying, it's all about managing expectations. We have the same rules for eating at our table at home as we do eating out, and our son knows that if he's not going to follow the rules there will be consequences.
At Disney we were never the only one to have a young kid in the restaurant - so your group won't be alone. Cast members were always very nice to us, our son included. We had small quiet toys on hand to entertain him while we were waiting on our server or food, and we kept an iphone on hand for emergency situations (we only needed to use it once thankfully).
We ate around our normal dinner time when we could, and we would just order his meal with our beverages so it would come out sooner. One of the chefs even took the time to pre-cut all of his chicken tenders for us (without us asking) before it came to the table - that was a wonderful surprise! Sure he was done eating way before we were sometimes, but that's when the toys came out if we needed them. We also have a rule at home that we don't leave the table until everyone's done eating, so again, our son knew what was expected of him while we were eating at all times.
I know this last part may fall under bad parenting, but never underestimate what a dessert can do for motivating or rewarding good behavior. For a few meals we knew we wanted to order dessert, so we let him know that for those times, he could share with us as long as he listened to us and followed the rules. Hey, it's vacation, right? A little ice cream for good behavior never hurt anyone. We were always going to share the dessert, because really our son does very well eating out, but it's nice to have some added insurance when everyone is on overload, am I right?
Anyway, I hope that helps. Really, if things do go wrong, you always have the 24 window to cancel other reservations and hey - it's not like you'll be the first person to have a screaming kid at Disney World.
