I was tired of never getting the reading for 判子 right so I figured I’d look through all the Wanikani vocab to see when 子 is し and when it is こ. This is what I found, thought I’d share it in case anyone else might find it useful, and to see if anyone has any better rules…
子 usually uses the onyomi reading “し” for jukugo (multiple kanji, no hiragana) words, as you’d expect [and for “お菓子” (おかし) which is honorific prefix + jukugo].
子 uses kunyomi reading “こ”…
…when you’d expect to use kunyomi (single kanji, kanji+hiragana together).
…whenever it is used at beginning of a word (except for one case - see below).
These rules will get you through 30 out of 42 vocab words containing 子 on Wanikani.
Exceptions (this is a complete list of exceptions to the above rules in the Wanikani vocab):
王子 - おうじ - Prince
障子 - しょうじ - Japanese Sliding Doors
し at beginning of word:
子孫 - しそん - Descendant
こ/ご in apparent jukugo words:
息子 - むすこ - Son
判子 - はんこ - One’s Seal
親子丼 - おやこどんぶり - Parent And Child Bowl
穴子 - あなご - Conger Eel
迷子 - まいご - Lost Child
双子 - ふたご - Twins
す (alternate onyomi reading - two are “chair” so there are really only 2 cases here):
椅子 - いす - Chair
車椅子 - くるまいす - Wheelchair
扇子 - せんす - Folding Fan