On vegan and vegetarian parents

From The Guardian,

“Prof Allen conducted a study of impoverished children in Kenya, and found that adding as little as two spoonfuls of meat a day to their starch-based diets dramatically improved muscle development and mental skills.

. . .

Prof Allen was especially critical of parents who imposed a vegan lifestyle on their children, denying them milk, cheese, eggs and butter, as well as meat. ‘There’s absolutely no question that it’s unethical for parents to bring up their children as strict vegans,’ she said.
. . .

However, the British Dietetic Association said the study looked at impoverished, rural children with a poor background diet low in essential nutrients such as zinc, B12 and iron, and its findings were not applicable to vegan children in the developed world.”

Now assuming this is true, why doesn’t the ethical quandry hold for parents who don’t force their children to exercise, etc.?