We also ate at a few buffets where DS could eat free as others have said. For counter service, we would bring a small can of the pop-top Del Monte canned vegetables for him to eat and then I would share my meal with him. If it wasn't enough food for me then I would either eat something like a granola bar, too, or order an additional children's meal.
If you eat at a non-buffet table service restaurant, you can either feed your child off your plate or order a children's meal. There isn't a discount on the children's meal for being under 3 years old, but I personally think it is appropriate to bring food into the restaurant (like applesauce, vegetables, crackers) for a child that young if you want.