@lionfromnorth: Uh, 'this' probably:
https://mangadex.org/chapter/802607/15
They likely could tell the seed won't grow (hence why they asked if Dias would be sad)
And also that they have the power to make them grow but likely were told by their parents to never reveal it (hence the question about keeping a secret)
The former is probably something akin to how Aruna explained to Dias how her tribe looked those shiny jewels as mere stones because they don't have magic power in them, just a different type of extra senses.
The latter were likely some kind of rare/thought to be lost sort of power, and the parents told them to keep it quiet to avoid them being exploited.
Given the long-eared mouse tribe came from the desert, I'm guessing the mouse lady will give the explanation on why the seeds won't grow.
And then the twins will likely offer their power after some seeing Dias pushing himself so much.