If George PRATT died after 1905, you should be able to get a death record for him from Dept. of Health, 523 E. Capitol, Joe Foss Bldg., Pierre, SD 57501. That is supposed to name his parents.
Sometimes you need to research the more current members of the family first. The SD Births site [state.sd.us/doh/vitalrec/birthrecords/index] has nine children for Myron. SSDI indicates that son George died in Nov. 1964 in Illinois, & Lavern (1 Jan. 1901 - Jan. 1967, Friendship, WI). You might find the girls on SSDI by inserting exact dates of birth with their first name only for an advanced search. BLM.GLO gives the legal description of the homesteads of George & Myron: 107-65, land in Sections 4, 5, & 9. Those would be west of Wessington Springs. You can order application and proof papers for the homesteads from the National Records Center, Washington, DC.
Since you know where Lavern was born, you should find Myron on the 1900 Census there. That would show where both Myron and his parents were born. If your local public library participates in InterLibrary Loan, they can get microfilm for you. If not, go to a Family History Center of the LDS Church.
The State Historical Library, 900 Governors Drive, Pierre, SD 57501, has an 1885 State Census that you can get on ILL, too. They also have WPA Cemetery Project. If you think you know where people were buried before 1940, they might have a record.