I wanna point out that, when kids write a "Letter for Santa", the parents are the ones that end seeing the letter, and get gifts based on that.
My parents did that too.
So it's not a thing of reading comprehension, it's just a thing of knowing how it is to write a card to Santa and getting gifts on Christmas.
The Mom saw it, thought that the daughter liked Octopi, and ended buying her one as a Pet, which ended badly.
A silly Mom and a Kid that didn't think about how a REAL Octopus would be and feel when actually having one.
Also, as for her eating Takoyaki here...
Apparently, Machine Translation can read it as "Yakoyaki without Octopus is Delicious", so my theory here is that It's possible that it actually said that and the MTL translated it as it was.
But the Human Translator went "Wait, that doesn't make sense. I must be reading it wrong. Let's change it to a version that does make sense, and later I'll do a bit of Japanese practice since I may be having some things wrong.". Possibly having not watched the Anime where we see that Kaoruko specifically asks for Yakoyaki without Tako.
Which would be understandable, not all Manga Readers will watch the Anime and not all Anime Viewers will read the Manga.