Is 黄毛小儿 a sort of idiomatic phrase? Literally it would mean a "yellow haired child."
I saw it on a TV drama, but from the child in question was definitely not yellow haired. Since the speaker was speaking rudely, I'm assuming 黄毛小儿 is supposed to have a derogatory meaning and probably means something like a naive or foolish child.
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).
黄毛小儿 is typically focusing on one's immaturity, ignorant or inexperience, instead of foolishness. Its connotation can be various based upon how you use it.
For example, 你就是一个黄毛小子/黄毛小儿 draws an analogy between 你 and 黄毛小儿. It means you are a kid still with foetal hair, implying that you are still young and you are unable to understand or deal with sophisticated things, or you are ignorant about something. It can be an insult or derogatory when you refer to a young inexperienced guy as 黄毛小儿.
On the other hand, if you say: "你这个黄毛小子!", it means that you are naughty, making troubles, and etc. just like a young kid normally does.
