The children are fitted out in American Civil War uniforms.
Edouart's Photographic Gallery was at this address 1865-1867 and so this photograph may have been taken towards the end of the war (9 April 1865) or soon after.
Peter E. Palmquist & Thomas R. Kailbourn, 2000, Pioneer Photographers of the Far West: A Biographical Dictionary, 1840-1865, (Stanford, CA: Stanford University Press)