Possibly same person without parents!
Jennie C Gray in household of Henry G Thompson, "USA Census, 1870"
Event Place: Michigan
Gender: Female
Age: 6
Race: White
Birth Year (Estimated): 1863-1864
Birthplace: Michigan
Page Number: 40
Household ID: 322
Line Number: 31
GS Film number: 000552168
Digital Folder Number: 004271417
Image Number: 00272
Household Gender Age Birthplace
Henry G Thompson M 63 New York
Sarah M Thompson F 56 Connecticut [? nee Woodruff]
Sophroney Thompson F 24 Ohio
Elisa S Gray F 14 Michigan
Jennie C Gray F 6 Michigan [? Niece of Sarah Thompson]
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Jennie C. Gray in household of Harvey E. Kellogg, "USA Census, 1880"
Event Place: Centerville, Appanoose, Iowa
Gender: Female
Age: 16
Marital Status: Single
Occupation: At Home
Race (Original): W
Ethnicity: American
Relationship to Head of Household: Niece
Birthplace: Michigan
Birth Date: 1864
Father's Birthplace: Ohio
Mother's Birthplace: Ohio
Page: 554
Page Letter: D
Entry Number: 540
Affiliate Film Number: T9-0326
GS Film number: 1254326
Digital Folder Number: 004240671
Image Number: 00193
Household Gender Age Birthplace
Self Harvey E. Kellogg M 47 Michigan
Wife Sophrona H. Kellogg F 35 Ohio [Mother Sarah Thompson - see 1870 census]
Dau Inez Kellogg F 7 Iowa
Dau Francis S. Kellogg F 5 Iowa
Mother-in-law Sarah Thompson F 68 Connecticut
Niece Jennie C. Gray F 16 Michigan
Other Francis Mitchel F 18 Missouri