Since not much information is available now, let's join forces and try to put together a list of all adoptable kids and followers that can be turned into stewards. I think many will find such a list helpful, at least until the information is available on uesp and wikia. =)
Here's a list I found: Alesan - Dawnstar Blaise - Katla's Farm Francois Beaufort (boy) - Honorhall Orphanage Hroar (boy) - Honorhall Orphanage Lucia (girl) - Whiterun Runa Fairshield (girl) - Honorhall Orphanage Samuel (boy) - Honorhall Orphanage Sofie (girl) - Windhelm Britte - Rorikstead - IF Lemkil is dead. Gralnach - Heartwood Mill - IF Grosta is dead. Earlier quest to find dead father may also be necessary. Sissel - Rorikstead - IF Lemkil is dead. Skuli - Old Hroldan - IF Eydis is dead. Svari - Solitude - IF Addvar and Greta are dead. Dorthe - Riverwood - IF Sigrid and Alvor are dead.
To add to the lists, simply post the name of the child/follower you've found to be adoptable/stewardable. Even if the name is included in the above lists, feel free to confirm it. No one's really sure how credible these lists are. Also feel free to provide any additional information you want (any conditions? gender? race? possible glitches? etc etc).
I've only tested Lucia and Lydia so far, but yeah, they are confirmed.