It's not just racial diveristy, but general appearance. All of the children share the exact same head mesh. It's creepy.
That is also true. The problem comes down to the fact that children have no FaceGen data, so they are all identical in the face, literally. It also makes it that much harder for modders to do anything related to new child races. Note that there are no mods in existence for Skyrim that actually add new child races, at least none that I'm aware of.
Back on the topic, it's doubtful that they would add an NPC that exists in a separate DLC as a marriage option for Hearthfire. It doesn't even seem likely that they will add any additional marriage options at all, since no DLC as of yet has added any new candidates as far as I'm aware.
[Intelligence] Oh. So the fans complained and they get. So all we have to do is complain
The fans complain about things wanted but never got, fans complain when said things are added in because they're unoriginal ideas, Bethesda adds in things that are not in the request lists, going the "do something completely new and original and not anything demanded for" route for the fans sick of seeing the requested features getting added in, then fans complain that things they wanted aren't being added in.
Yeah, that's the industry for you. They have to figure out the best ways to satisfy as much of the target audience (particularly fans, but not limited to fans) as possible, which is generally through a combination of complaints/requests and stuff that isn't even being asked for, the latter of which is essential for keeping things interesting.