"Many children are short."
Translation:Viele Kinder sind klein.
Both are "klein" in German.
If you tell me that "sie hat ein kleines Kind", I'll assume you're referring to the child's age (maybe 1-6 years old?).
If you tell me that a child is "klein", it depends on context. I might imagine the child as "young" or as "short". "Sein Kind ist noch klein" = his child is young / only a few years old; "Das Kind dort ist klein" = the child over there is short; "Er ist zu klein für die Achterbahn" = probably he's too short to ride on the rollercoaster; "Er ist zu klein für diesen Film" = he's too young to watch this movie.
The word "klein" doesn't give me any mental image of the child's weight / "width". Same goes for adults: a "klein" adult is short, a "groß" adult is tall, not saying anything about their weight either way.
If you want to tell me that a child is "tiny all over" (frail-looking), I can't think of an actual word for that, outside local dialects with funny words.