I suspect that this might be a portrait of Giuseppe Garibaldi (1820-1884) - and not an image of a civil war solder.
See for example these portraits of him wearing a similar shirt and carrying a similar sword."
Regarding the photo by William Engand near Niagara Falls, the woman is very likely to be his wife Rosalie. He had a habit of parking her on glaciers, rickety bridges and alpine meadows for foreground interest. The young man is probably Louis England, their eldest son. I'm not sure who the little girl would have been. The England clan lived in an enclave in St James Square, Notting Hill, with at least three houses. They worked as a family firm making stereoviews. This would have been an early working trip (1864) but even with later trips, the Englands did not seem to take their younger children with them.
A younger boy around 12 or so, possibly Walter, appears in the later Swiss and Italian Alps series, but not a child this young.