I didn't understand... what is the mistake ??

Mistake of sacrificing the white haired kids? ┐(´(エ)｀)┌

The text framed it like the mistake was that the white/silver haired children are sacrificed so that the other ones can become stronger and get more magic. But that is the mistake because turns out that the silver haired ones deserve the other ones being sacrificed. I assume they have more magic potential in later years or something. (〜￣△￣)〜

That is the only sentence that I understood.

thanks guys !