I've seen enough female slash fanart to know that those kids aren't adopted.
Note: see Kigo family fanart, Blossom/Bubbles family fanart
I thought the same thing at first, the kids have too many traits from Korra and Asami to just be "adopted". But just this once, I'm gonna give the artist the benefit of the doubt, since it's the best Korrasami art I've ever seen
And this is the second

Lawd have mercy on my soul