If you go back and look at the 1900 census you will see that also living in the MOXLEY household is mother-in-law Maggie M. LONG.I'm assuming that the 1870 date you wrote in your post as Shelton's birth year is a typo.In 1900 he says he was born in Aug 1876.
A TX death record for Viola MOXLEY lists her parents as Sam LONG and Maggie BALLEY.
There's a TX marriage record for M.M. WOLF to S.H. LONG on 27-Sept-1881 in Hopkins Co, TX.
I suspect that Maggie BALLEY WOLF LONG is the mother of both Viola and Shelton.
In Hopkins Co. in 1880 there's a widowed Margret WOOLF living one census page away from Henry and Susan WOOLF.She has two sons, John, age 12 and William, age 6.I was expecting to find a 4 yr old Shelton in this family.I am only looking at a transcription of the census.Perhaps if you looked at the actual image you might find him there.It's possible that Henry WOOLF was related in some way to Maggie's first husband.
Since 12 yr old John WOOLF is listed as born in AL, I checked the 1870 census and found this family in Talladega Co: WOLF, Mahala L. age 28 b. AL Margret M. age 22 b. GA John H. age 1 b. AL BAILY, Zilla age 65 b. SC
There's a TX death record for J.H. WOLF b. 1869 in "Taladago Co, AL" d.15-March-1936 in Cumby, Hopkins, TX.Spouse:Emma WOLF, Father: M.L. WOLF, Mother:Maggie BAILEY.
If you know Shelton's date of death, have you tried sending for a copy of his death certificate?
There's a user submitted tree on familysearch that shows a son M.L. WOOLF, born 1842, to Henry W. WOOLF and first wife.