Well, they were not easy to find, but I finally tracked them down in the 1880 US Census, living in Mulberry, Clay Co., Kansas.
Anthony (enumerated as "A.D.P." Ferguson, 44, was a grocer, born in Canada of Canada-born parents; his wife, Laura E., 36, was born in Indiana of NJ-born parents. Future census records of two of their children also record their parents as born in Canada and Indiana, respectively. Their children, enumerated in the 1880 census with them include (note extra daughter):
Mary L., 18 (b abt 1862) Illinois
Margareth R, 15 (1865) Illinois; she's not on your list
Ellen M., 13 (1867) Illinois
Dora E., 9 (1871) Kansas; indicating a move btw 1867 and 1870 from Illinois to Kansas
John C., 6 (1874) Kansas (out of order from your list)
Adel M., 3 (1877) Kansas
Julie A., 1 (1879) Kansas
Lepha is not listed on this census; presumably born after the census was taken (3 Jun 1880)
The parents and the first three children should be a family unit in the 1870 census, but I could not find them anywhere. If they were in transition from Illinois to Kansas, the census taker may not have found them.
Based on the oldest child's approximate birth year (1862), I'd guestimate that Anthony Ferguson and Laura Hagen married about 1861 probably in Illinois. I could not find either of them single or married in the 1860 census.
One possibility for Laura in the 1950 census is the 10-year-old daughter of John Agan in Washington, Daviess, Indiana. The census image is difficult to decipher.
I also did not have much luck finding them in the 1900 census -- Anthony and Laura, that is. Which is too bad, as it would have provided an immigration year for Anthony. (And subsequent search of Ancestry's immigration records brought up nothing.)
I did find (in 1900 census) Ella and Edmond Todd married 10 years in Grant, Iowa, with a number of children. And I found Dora married to Frank Hammond with dau Vera in Dixon, Sumner, Kansas. Both of these 1900 census records indicated the women's father was born in Canada; mother born Indiana, matching the 1880 census information.
I was unable to easily find any of the other Ferguson children in the 1900 census.
Of possible interest: I found Lepha (rare name) Ferguson in a 1906 city directory for Kansas City, MO, living at 1409 Locust, and working as a waitress.