The 1900 census Seneca, Newton co, Missouri census has:
DeWitt Clinton BUTTON abt age 74 b OH; CT?; PA?
Mattie (RHEAM) BUTTON abt age 31 b KS;PA; TN
Addie Moon dau? age 10 b Sep 1889 MO; IL; ARK
Jessie H Moon dau? age 8 b Jan 1892 MO; IL; ARK
Ivon or George Moon son? age 6 b Dec 1894 MO; IL; ARK

next door lives Floyd and Estella Williams- perhaps Mattie's mother who has remarried.

Does anyone know these MOON children?

Mattie RHEAM was about 27 when she married DeWitt Clinton BUTTON who was a widow in his 70's. He died the year after this census on 21 Oct 1901 in Seneca, Missouri.
Mattie died 5 Dec 1944 in Joplin, Jasper, Missouri.

Mattie (RHEAM) BUTTON's obituary published Thurs Dec 7, 1944 in the Neosho [Missouri] Times:
"Mrs. Mattie BUTTON, 75 years old, died Tuesday night at St. John's hospital after a brief illness. She was a former Seneca resident before moving to Joplin about ten years ago. Surviving are two daughters, Mrs. Pearl WILLIAMS of Midland, Tex., and Mrs. Jessie HICK of Santa Monica, Cal; a brother, W. H. RHEAMS of Tulsa, and a sister, Mrs. John LEONARD of Springfield [Missouri]. Funeral services were held at 2:30 o'clock this afternoon at the Buzzard funeral chapel in Seneca [MO]. Rev. Mr. Ward of Joplin officiated and burial was in the Seneca cemetery". ---end---- [note: she is buried next to DeWitt C Button, her husband in Seneca Cemetery, Seneca, MO].