The newspaper articles and the chapter in the 1861 California State Statutes mention slave children in Missouri.
The affidavits from Zachariah Simpson's probate file say that Zachariah was a slave from Kentucky and that his master's name is James Simpson.
William Jones testified in the corner's inquest that Zachariah was born in Madison County, Kentucky and he was 42 years old. Zachariah would have been born about 1818.
I started in Kentucky, looking for Simpson's that owned at least 5 slaves in 1820. The affidavit of William Jones said that he was a slave along with Zachariah, his brother Squire and their parents Timothy and Gracey Jane in Kentucky and their owner was "Judge Simpson."
In 1820 there was only one man named Simpson in Madison County, Kentucky that owned at least 5 slaves. Richard Simpson had 26 slaves.
In 1830 there were more Simpson's in Madison County that owned slaves:
Richard Simpson owned 14 slaves
Martha Simpson owned 25 slaves
Duke Simpson owned 16 slaves and there was 1 free colored man
The affidavit of Sallie Drisdom, the wife of Squire Drisdom, said that Zachariah's master's name was James Simpson. It isn't until 1850 that we see a James Simpson who owns slaves in Kentucky. There is one James Simpson who lives in Clark County and owns 9 slaves. There is another James Simpson who lives in Marion County and owns 19 slaves.
Nothing is falling into place with search results from Kentucky.
Because the information in the Senate bill mentions Missouri, that is where I looked next.
In 1820, there are no Simpsons in Missouri that own slaves.
In 1830, there is one man named Robert Simpson who lives in St. Louis and has 12 slaves.
In 1840, there is a Richard Simpson who lives in Jackson County, Missouri who owns 12 slaves. There is a James Simpson who lives in Warrensburg, Johnson County, Missouri who owns 12 slaves.
And finally there is a man named Duke Simpson who lives in Jackson County, Missouri and owns 19 slaves. Is this the same Duke Simpson from Madison County, Kentucky in 1830?