"Assignment # 9 (A)"

The weapon was referred to in this word-of-mouth report as gensbi bakudan-the root characters of which can be translated as “original child bomb”
The weapon was referred to in this word-of-mouth report as gensbi bakudan-the root characters of which can be translated as “original child bomb” (p62 Hiroshima)
In this sentence, “original bomb” means that the weapon “B-29” is used for the first time in the world ; however, I can not exactly get that why people says "child bomb."