Before we went to a buffet with our first son, I also was concerned about that "off of your plate" statement. We've never had to actually do that, as our boys have always had their own plates at the buffets.
Up until this coming trip, we'd actually plan more buffets partially because we've had someone under 3 on most of our trips. Always have been offered a complimentary drink, which I was always expecting to pay for, especially since we would always get milk.
Last year our server would always ask about our just turned 2 y/o, except for one time. We were charged, but that was quickly taken care of. Couldn't blame a server for thinking he's older than he is. He's only a couple pounds lighter and couple inches shorter than his brother who's about 3.5 years older.
At menu dining restaurants we've either ordered OOP for our under 3's or we've split meals, and the servers actually brought an extra plate for us, or split them in back before bringing them out.