It’s interesting that something the author chose to name it “cat powder chocolate” and it acts like a catnip to them, keeping in mind that actual chocolate is poisonous to real cats.
I suspect the SFX that was translated as "roll" on the last page was ゴロ, which is also used for cat's purr. Since they're sleeping, I'd assume they're purring.
Huh so they do really lose their tail after growing up? Or maybe they just learn how to hide it with magic? I'm leaning a bit on the latter because Lucola's tail popped out after taking that choco cat nip, maybe because she started losing control on her magic or something.