Meaning of the expression 黄毛小儿
naive or foolish child


This is right (except maybe foolish is a bit harsh). 黃毛 literally refers to foetal hair (alternatively 胎毛), the hairs on a newborn that will fall off later. The hair is unpigmented which contributes to its name (in contrast to the black colour found in most East Asians). The metaphor here is young/immature/inexperienced/silly.

Similar phrases include 黃毛丫頭 (specifically for girls, as 丫頭 refers to the hairstyle of girl servants in the olden times).