According to the 1850 Census, Robert D Donnell was the first child born in MO...Mary Ann was b. TN

1850 United States Federal Census about John Donnell [13 August 1850]
Name: John Donnell
Age: 41
Estimated Birth Year: abt 1809
Birth Place: North Carolina
Gender: Male
Home in 1850 (City,County,State): District 71, Polk, Missouri
Household Members:
Name Age
John Donnell 41
James Donnell 39, TN [sic-Jane]
Mary A Donnell 19, TN
George W Donnell 18, TN
William M Donnell 17, TN
Margaret R Donnell 15, TN
Robert D Donnell 14, MO
Jane H Donnell 8, MO
Nancy C Donnell 7, MO
Sarah C Donnell 6, MO
Columbus W Donnell 5, MO
Thomas M Donnell 3, MO
Winfield S Donnell 1, MO
Robert Wells 19 , MO, [sic-Wills]
Cyntha D Wells 12, MO
Samuel A Wells 9, MO
John Wells 7, MO
Source Citation: Year: 1850; Census Place: District 71, Polk, Missouri; Roll: M432_411; Page: 5; Image: 10.
Source Information: Ancestry.com. 1850 United States Federal Census [database on-line]. Provo, UT, USA: The Generations Network, Inc., 2005. Original data: United States of America, Bureau of the Census. Seventh Census of the United States, 1850. Washington, D.C.: National Archives and Records Administration, 1850. M432, 1,009 rolls.