Searched for Robert A. Lankford on censuses using information from the 1920 census. I think I have located him with his father in Tennessee on the 1870 census. From the Information on the 1870 Tennessee census it appears that the parents lived in Alabama prior to Robert's birth.
The 1870 Giles County Tennessee census records the household:
A.J. Lankford 38 Farmer; birthplace too light for me to read.
M.J. 26 female born Alabama
Henry 16, born AL
R. A. 4 born Tennessee
P.James, 60 Female widow b.TN
F. Gallien, 16 female b. AL
W.R.Gallien, 12 male. b.AL
M. Gallien, 8 female, b. AL

The Gallien where found on 1860 census in Lawrence Co. AL.
The name was spelled Galyean. A.J.'s wife may have been a Gallien/Galyean.

A.J. was born circa 1832 and his full name is probably Andrew Jackson Lankford as this name was adopted by many families for their sons born at that time..The Indian removals, etc. by Andrew Jackson.

I also looked at the Find A Grave entry for Robert A. and viewed his photo. It would stretch my imagination quite a bit to determine that he was Native American with such Anglo features. No flat face nor wide nose etc.

Note that the 1910 census for Robert contained an error.
his place of birth was listed as unknown and he was also and importantly referred to as a widower which is not surprising at 43-years of age. He is listed as a border and I am assuming he dis not provide the information for his entry which is not unusual and family researchers are warned about this frequently by the professionals.

Robert could have had a previous family and the children could have been living with relatives or out on their own if they were about 20-yeers of age if Robert married young.

Regarding Lankford/Langford Native Americans. The only entry for a married man on the final Dawes Rolls is for Thomas N. Lankford and that was "by marriage." His wife and children are listed as Native.
No listing in any tribe for a Native American Lankford.

As for Robert A., he just does not fill the bill and you may be wasting precious research time pursuing this avenue when his parents need to be tracked; and,they being in the southern states where records are scarce will chew up all the years left in one's life.

Happy Hunting.