Why not children’s faces??
That was my answer too. I am reporting it (Mar 2018)
It now says "children’s faces" in the answer above (June 2018), so I think they have fixed this one.
But sometimes "faces of children " would be appropriate. It should be accepted.
Why not "faces of children"
Also correct English, but more formal.
Please. Why za?
For plural nouns of the U/N class you use z as a prefix with the a of association. I got this wording (which I revised hopefully correctly--- not sure) from the tips on Noun classes: U/N Noun Class on the last page.
I think children should work too