With a white chicken in the picture for comparison, I think I have a better idea of her color. I kept thinking she looked white or almost-white in the initial photos, but now I think that chick looks lavender.
So the chick probably does not have Dominant White, but does have the lavender gene (inherited from the father and from whichever hen is her mother), and probably does come from the parents you showed in the first post.
I'm thinking they are not actually pure Cuckoo Marans
If they are the parents of this chick, and you hatch more chicks from them, you've got a fairly good chance of getting more lavender chicks (about 1 in 4).