Q: "--- why would 2 older brothers and 3 younger sisters would be born in Tennessee, and John J. Bennitt being born in South Carolina?"
This is not unusual. The census indicates that they owned no real estate, and they were most likely to be share croppers. Generally, southern share croppers rented a house for one year and moved frequently. Crossing state lines meant nothing. Sometimes when things got tough they moved in with relatives that could have lived in another state. It is noted that Lucy was born in South Carolina. She may have gone "home" for the birth of a child leaving her husband home to take care of the 4 and 9 year old boys.
Q: "Wonder if Lucy Bennitt and her children (possibly husband) are on the 1840 census?"
The 1790 - 1840 censuses only listed the head of household by name. Other members of the household were enumerated by sex and age group - not names. The only exception to this was in the 1840 census that provided a column for the names of pensioners for Revolutionary War or other military services living in the household. Without knowing the state and name of the head of household and the total number of people living in the house, whether related or not, would make this question difficult to answer.