William married Mary __ before 1820. Six children were named in William's probate records when he died in 1841: Harriet, Eliza, Mary Anne (Polly), Jacob, Dodson and Helen (Ellen).

Note: An additional child, William, has been identified though DNA testing. Two descendants of William (Jr.) were compared to several descendants of Eliza, Jacob, and Dodson and the data clearly shows that the two descendants of William's fall well within the 3rd-4th cousin range.
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According to his land record in February 1841 he had 9 children and no wife.
Original record located at the National Archives in Washington D.C.
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From "Vital Records from Chicago Newspapers, 1840-1842", p.22

Daily Chicago American 1841

March 10, Died
On the fifth inst. at Clybourneville, Kane County, Ill., William VAN DEVENTER, aged about fifty."
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1825 New York State Census
The Town of Chautauqua, NY
KEY:
A. Name of head.
B. Number of male persons in family, ind. head.
C. Number of female persons, in family, ind. head, if female.
D. Number of males subject to militia duty.
E. Number of males entitled to vote.
F. Number of male aliens, not naturalized.
G. Number of married females under age 45.
H. Number of unmarried females 16-45.
I. Number of females, unmarried under 16.
J. Number of births in year preceding.
M-Male
F-Female
K. Number of deaths in family during year preceding.
M-Male
F-Female
L. Number of acres improved land occupied by same family.

VanDeventer, William 1 2 1 1 0 1 0 1 4 4 1 23 64
Lyon, Joseph 4 3 1 1 0 1 0 2 F 25 7 21 6 10 17 40
Lyon's data is included because according to the Kane County, IL history books William VanDeventer went with Joseph Lyon to Kane County, Illinois in 1834.
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1830 Federal Census
William Van Deventer Home in 1830 (City, County, State) Chautauqua, Chautauqua, New York Free White Persons - Males - 30 thru 39 1 Free White Persons - Females - Under 5 4 Free White Persons - Females - 5 thru 9 1 Free White Persons - Females - 30 thru 39 1 Free White Persons - Under 20 5 Free White Persons - 20 thru 49 2 Total Free White Persons 7 Total - All Persons (Free White, Slaves, Free Colored)
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1840 Federal Census
Name: Wm Vanderenk [Wm Van Deventer]
Home in 1840 (City, County, State): Kane, Illinois
Free White Persons - Males - Under 5: 1
Free White Persons - Males - 5 thru 9: 2
Free White Persons - Males - 40 thru 49: 1
Free White Persons - Females - Under 5: 2
Free White Persons - Females - 5 thru 9: 1
Free White Persons - Females - 10 thru 14: 1
Free White Persons - Females - 15 thru 19: 2
Persons Employed in Agriculture: 1
Free White Persons - Under 20: 9
Free White Persons - 20 thru 49: 1
Total Free White Persons: 10
Total All Persons - Free White, Free Colored, Slaves: 10