noun
exceptionally healthy child; prize-winning child in a health contest
Refers to a child recognized for outstanding physical health, often in the context of a school or community health competition. Can also be used figuratively to describe someone who is the picture of health.
彼女は小学生のとき、健康優良児に選ばれた。
She was selected as a prize-winning healthy child when she was in elementary school.
That kid is really the picture of health.