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):
Rendaku (じ):
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
I did a quick search looking for words containing 子 on jisho.org and found other exceptions to your general rules. It seems like 子 is a particularly tricky kanji because there seemed to be a few that I came across that one normally sees written in kana:
団子【だんご】
茄子【なす】
硝子【ガラス】
There were several that came up as alternative readings/forms for other words:
For 砂 → 沙子【いさご】、砂子【すなご】
For 羽 → 羽子 【はね】
For 梯子【はしご】 → 梯子【ていし】
For 杏 → 杏子 【あんず】
And then others that fall under as being exceptional:
様子【ようす】
餃子【ギョーザ】
Another thing I noticed is with regard to the kana guideline mentioned in the OP. The honorific お/御 doesn’t seem to count:
お菓子【おかし】
御子【みこ】
One tendency I did notice, though it wasn’t 100% consistent was that kanji compounds referring to a child usually used こ/ご as a reading rather than し.
Obviously this was a very superficial search, and I tried to include examples of common words (exception 御子). 子 is definitely a formidable kanji.
At least we know the what to look for in WK when doing reviews. Thanks for the interesting tidbit.
This doesn’t even show up in 大辞林. Looks like this is one of those "Someone used it once ever 500 years ago, so lets make sure it’s in EDICT.
Anyway, 調子 is at the end of a word as し. I’m not sure if it’s the case for this, but one piece you’re missing is like in 障子, the 子 at the end of the word is an attempt at emulating the Chinese style that exists in a number of words like 餃子 (another type of reading missing from your list) where the 子 only exists as a “meaningless” noun suffix.