12 years of on-line and 40 of off-line genealogy tell me that with names as rare as Troy and Dewitt, "Illinois" is not entirely correct and these are your people. The chance of there being another Dewitt & Rachel with a son Troy, who skipped the census in 1910 and 1920, is remote. Rachel's maiden name may not have been Ragsdale, either.
The possibilities are
1) Your source was wrong.
2) She was a Ragsdale, she married a man named McManus at age 16, and he died, deserted or divorced.
3) She married a man named Ragsdale after Dewitt died, and the source was garbled.
4) None of the above.
1910: Poplar Bluff Ward 3, Butler, Missouri; Roll T624_769; Page: 4B; Enumeration District: 13; Image: 1052.
(Household, name, relation, sex, race, age, approx birth year, status, birthplace, father's BP, mother's BP)
76 Thompson Dewitt Head Male White 26 abt 1884 Married Missouri Missouri Missouri
76 Thompson Rachel Wife Female White 19 abt 1891 Married Missouri Missouri Misouri
76 Thompson Troy C Son Male White 6/12 abt 1909 Single Missouri Missouri Missouri
They have been married 2 years. He is a laborer in a stone factory or stove factory.
1920: Poplar Bluff, Butler, Missouri; Roll T625_909; Page: 5A; Enumeration District: 10; Image: 271.
Dewitt Thompson 36
Rachel Thompson 30
Troy C Thompson 9
Lloyd Thompson 6
Lewis Thompson [1 and 6/12]
1930: Epps, Butler, Missouri; Roll 1179; Page: 2A; Enumeration District: 10; Image: 680.0
Chaley C Scott 28
Rubbie W Scott 28
Agnes P Scott 8
Ray S Scott 5
Chealey Scott 3 6/12
Ronald F Scott 1 4/12
Wilma L Scott
Hellon Johnson 14
Laney Maye 19
Troy Tompson 20
Troy is a boarder with the Scotts. His occupation is Labor, truck helper. Laney Maye is a truck driver, so odds are they are partners.
I can't find Dewitt or Rachel in 1930. He is probably Dewitt Clinton or John Wesley Dewitt, and used his other name in 1930.
Missouri Marriage Records, 1805-2002
Name: Dewitt Thompson
Birth Date: abt 1884
Marriage Date: 18 Apr 1908
Marriage County: Butler
Spouse Name: Rachael Mc Manus
Spouse Age: 18
I can't find Dewitt Thompson, Rachel McManus or Rachel Ragsdale in 1900.
> where can i find the thompson family tree
"Thompson" is probably one of the 20 most common names in the US; it is certainly in the top 100. It means "Son of Thomas". There are thousands of unrelated Thompsons, each with their own family tree.