This child is probably a boy. The part in the hair is slightly to the side of center, and the dress is not so frilly as many girl’s dresses from the 1880s or so. He has a bit of baby fat still evident in his face, so I’m guessing the age of about 2-3, still within the range of wearing diapers as well. The cabinet card is the lower left of the third open page of the Dobb Long Book. This is the third dark green cabinet card in the album, supposedly much more rare than maroon and black cards, but I’ve yet to see a maroon one and now have four green. Go figure.
The photographer was Rothschild Company, located at State Street & Van Buren St in Chicago. This was probably within the A.M. Rothschild Department Store, the original building having been completed in 1881. At the time, it was a retail center of Chicago, with numerous mercantiles and dry goods stores lining the street. Later, in 1912, a new building was completed at the same location. A.M. Rothschild was eventually bought out by their neighbor, Marshall Fields in 1923. Various retailers occupied the space over the years, including Sears & Roebuck, but it now houses the downtown campus of De Paul University.
Given the color of the card and the opening date of the department store, I’m dating this to about 1885.