I'm just going to point out that this sets a troubling precedent. If one firm thinks it's legitimate to discriminate in favor of mothers with children, that suggests that being a parent is an acceptable basis on which to discriminate. It opens the door to people thinking about whether somebody is a mother in making hiring decisions. That's troubling on a lot of levels, and in some states illegal.

EDIT: apparently the source has been revised to make it clear that the point is just to look evenly at parents and non-parents; that's OK, but I'll leave the comment above in reference to the earlier version.