Children have growth spurts and plateaus and you really notice them slow down and leave a plateful of food when for the past months or years they have eaten a full plate and asked for more. It's mostly noticeable in boys in my experience and it even happened at about age 3 to my little daycare boy who has always asked for 3rds.

I have a little boy who has just turned 2 and he seems to have hit that plateau and is refusing to eat any meat lately. I give him tiny portions and if he eats it all I give him a little more. As long as I tell the parents what I fed them every day and what they ate or left behind so the parents might expect the child to be extra hungry in the evening I feel I'm doing my part and a good job.