Hi,

I couldn't find them in 1880 either. The only child that would have been born by the 1880census was Oscar in 1875. The only Oscar Ellis around that age lived with the following family:
1880 Census:

Name: James P. ELLIS
Age: 26
Estimated birth year: <1854>
Birthplace: Georgia
Occupation: Farmer
Relation: Self
Home in 1880: District 567, Henry, Georgia
Marital status: Married
Race: White
Gender: Male
Head of household: James P. ELLIS
Father's birthplace: GA
Mother's birthplace: GA
Image Source: Year: 1880; Census Place: District 567, Henry, Georgia; Roll: T9_152; Family History Film: 1254152; Page: 241A; Enumeration District: 70; Image: 0484.

Wife- Susan age 23 born GA
Son- Oscar age 8 born GA
Son – Walker age 5 born GA
Daughter- Sarah age 1 mos.

I couldn't find this family in 1900!! It could be possible that Susan, Walker and Sarah died and with Oscar left, James remarried Mary and had more children. Then James died before the 1900 census. ???

There was one Oscar Ellis, same age, in the Georgia death records: Maybe you could get the death certificate to see who his parents were.

Name: Oscar L Ellis
Death Date: 23 Dec 1960
County of Death: Bibb
Gender: M
Race: W
Age: 85 years
County of Residence: Bibb
Certificate: 32208

Good luck
Darlene