Because Sheridan and Custer said so altough they never saw the bodies themselves. The people who found them (Kansas volunteers all said that the Blinns stood near the location where the 7th cavaly had killed warchief Black Kettle)
That would be nice to believe but the Kansas Volunteers were not at Washita until Custer's second expedition to the river during which the bodies of Elliott and his men were recovered.
So, to be blunt about it, it is at the best second-hand information. I believe in legalese that type of information is called "hearsay."
It's called hersay when soldiers who were campaigning with the 7th cavalry in 1868 arrived in Black Kettle's burnt village, found goods from settlements in the area around the site, and ultimately found Clara Blinn's and Willie Blinn's bodies.
It is not hearsay at all. But it seems very hard for you to admit that your kind Cheyennes murdered in fact two innocents when they were fleeing their peaceful village.