Possible census records (I think Cyrus is incorrectly listed as a "daughter" in 1930):
"United States Census, 1930," Cyrus Cornwell in household of Sylvester Cornwell, Mineola, Nassau, New York
Name: Cyrus Cornwell
Event: Census
Event Date: 1930
Event Place: Mineola, Nassau, New York
Gender: Female
Age: 3
Marital Status: Single
Race: White
Birthplace: New York
Estimated Birth Year: 1927
Immigration Year:
Relationship to Head of Household: Daughter
Father's Birthplace: New York
Mother's Birthplace: New York
Enumeration District Number: 0147
Family Number: 391
Sheet Number and Letter: 16B
Line Number: 59
NARA Publication: T626, roll 1461
Film Number: 2341196
Digital Folder Number: 4661131
Image Number: 00425
Household Gender Age Birthplace
Head Sylvester Cornwell M 43 New York
Son Sylvester Cornwell M 12 New York
Daughter Cyrus Cornwell F 3 New York
Daughter Ruth Cornwell F 1 New York
"United States Census, 1940," Cyrus Cornwell in household of Sylvester Cornwell, Mineola, North Hempstead Town, Nassau, New York, United States
Name: Cyrus Cornwell
Titles & Terms:
Event: Census
Event Year: 1940
Event Place: Mineola, North Hempstead Town, Nassau, New York, United States
Gender: Male
Age: 13
Marital Status: Single
Race (Original): White
Race (Standardized): White
Relationship to Head of Household (Original): Son
Relationship to Head of Household (Standardized): Son
Birthplace: New York
Estimated Birth Year: 1927
Residence in 1935: Same House
Enumeration District Number: 30-234
Family Number: 168
Sheet Number and Letter: 7A
Line Number: 26
NARA Publication Number: T627
NARA Roll Number: 2691
Digital Folder Number: 005458663
Image Number: 00504
Household Gender Age Birthplace
Head Sylvester Cornwell M 53 New York
Wife Annette Cornwell F 40 New York
Son Sylvester Cornwell M 22 New York
Son Cyrus Cornwell M 13 New York
Daughter Ruth Cornwell F 11 New York
Daughter Annette Cornwell F 7 New York
Stepdaughter Joan Andrews F 12 New York