I have checked all cemeteries in and around Blackcreek township, Mercer County, Ohio and find no headstone for Mary "Polly" Downs.I did find her in the 1850 census.
Death and birth records do not start until 1867 in Ohio, so there is no death record.
However, I did find the following:
1850 Mortality Schedule for Mercer County, Ohio Persons who died in the year ending 1 June 1850 in Dist. 204 of Mercer County, Ohio
DOWNS, Hannah age 2 years female Born:Ohio Month died:August Died from: scarlet fever # of days ill:28 days
My Notes: 1. According to 1850 census, Dist. 204 is indicated to be Blackcreek township. 2. 1850 mortality schedule includes those who died between 1 June 1849 and 1 June 1850, thus, Hannah Downs died in August 1849. 3. I would say that Hannah Downs is a daughter of Mary Vian Downs, and Hannah was likely born in 1847. 4. I also did not find a headstone for Hannah Downs. 5. There is a high probability there is no headstone for Mary Downs if she died in Mercer County, Ohio. 6. Have you considered that perhaps Mary Downs died on the way to Missouri or died shortly after arriving in Missouri?