DD1 has always been fine to take out to restaurants, even when she was tiny.
My other 2 however, are too full of energy. I just couldn't put other diners through it. If we only have 1 often we'll take that child out for lunch, but then they have the focus of both parents.
Actually now that I say that a GF and I often take DD2 (2.75) out for a meal and she's pretty good until right near the end. We sit in a corner and both of us have bits and pieces in our bags. It helps that DD2 lives to eat lol, so new foods are exciting for her.

Dinner however is a different kettle of fish. My kids are really tired by dinner time and expecting them to hold it all together is unreasonable if you ask me. Find somewhere where they can run around. Two of our local clubs have great playgrounds, and that's where we (and all our friends with kids) tend to go.

That said, my parents loved restaurants and we went often as children. If they are exposed and it becomes normal, then they get better at it.