This vintage real photo postcard features a portrait of a sassy little girl and an apprehensive little boy. The children are adorably dressed and very expressive. The little girl’s picture should be in the dictionary under the word “sassy”. The postcard paper was produced by AZO sometime between 1904 and 1918. The postcard is unused and clearly once resided in a photo or postcard album. The image has excellent clarity.
THE CABINET CARD GALLERY 
They are beautiful and adorable! I bet she’s his little sister, and has him properly cowed. She must have become someone special as an adult. She certainly knows how to steal the show.
That sassy little girl is a boy. He’s wearing garments suitable for a boy at thst time.
A little girl would have been dressed as a girl, not [as in never] a boy.
What a disappointment for us girls everywhere who are mostly emancipated now, and would love to think she (!) was then, or at least her mother was.
This is not the first time I mistook a boy as a girl in a cabinet card photograph. The hairstyles and clothing at the younger ages are almost unisex. When I showed this photograph to my wife she, like you, said “that sassy little girl is a boy”. I should have listened to her.