noun
weak child; frail child; gangly child
Colloquial, often mildly teasing or self-deprecating term for a child who is physically weak, thin, or pale, likened to a bean sprout.
うちの子はもやしっ子で、すぐ風邪をひく。
Our kid is a weakling and catches colds easily.
子供の頃はもやしっ子だったけど、今は丈夫になった。
I was a frail child when I was little, but now I'm sturdy.
From もやし (bean sprout) + っ子 (child), referring to a child as thin and pale as a bean sprout. The exact origin is uncertain, but it is a colloquial coinage.