We ate 7 all-you-can-eat TS meals on our trip last summer and no one ever questioned my little guy eating off the buffet or family-style platters. He was 14 months old when we went. He ate more than my older one did at those character dinners...the older one of course was more interested in the characters than in the food! Minnie Mouse actually fed him some mac and cheese at LTT back when they still had characters there at dinner...and yes, I got it all on video! Priceless! He was, however, very interested in those Mickey waffles at the 2 character breakfasts we had!
I can't imagine anyone giving you a hard time at an a la carte TS either...the CMs were all so nice, plus it is the Disney policy that a child under 3 could share off an adult's plate. It says so in the dining brochure.