It looks to me as though this could be later in the 19th century, maybe the mid 1890s, when children's fashions became less restrictive. She doesn't quite have the leg-o-mutton sleeves happening, but they are long sleeves and not fitted as in previous decades. Her long hair worn down indicates that she was not yet of the age where long dresses went on and the hair went up - usually around 16-18 years old. I'm sure someone else will have much better and more accurate information on costume and hair than me, though!
For a definitive date on the photograph I would need to see it close up, but if I give you some advice on what to look for, you could do it yourself - let me know if you want to go this way. It could be one of a number of processes that were around in the late 19th century, and they can normally be pretty well identified using their physical characteristics.
Cheers
Prue
