Hi Jean,
I had a look on the BIVRI but couldn't come up with a birth for John. I wonder if Ann died after giving birth to her last child and David, (possibly using a middle name of James

) remarried Rachel! Stranger things have happened.
Anyway, just in case you're missing any of the children, here is what was on the BIVRI...
Children of David Sneddan & Ann Simson…
SNEDDAN, Ann Christening
Gender: Female
Birth Date: 29 Oct 1789 Birthplace: , Shotts, Lanark, Scotland
Recorded in: Shotts, Lanarkshire, Scotland
Father: David SNEDDAN
Mother: Ann SIMPSON
Source: FHL Film 102963 + Dates: 1787 - 1791
SNEDDAN, John Christening
Gender: Male
Birth Date: 13 Aug 1804
Christening Date: 19 Aug 1804 Recorded in: Shotts, Lanarkshire, Scotland
Father: David SNEDDAN
Mother: Ann SIMSON
Source: FHL Film 1066605 Dates: 1801 – 1825
SNEDDAN, William Christening
Gender: Male
Birth Date: 21 Jul 1806
Christening Date: 27 Jul 1806 Recorded in: Shotts, Lanarkshire, Scotland
Father: David SNEDDAN
Mother: Ann SIMSON
Source: FHL Film 1066605 Dates: 1801 – 1825
Children of James Snedden & Rachel Burt…
SNEDDAN, Helen Christening
Gender: Female
Birth Date: 28 Oct 1807
Christening Date: 22 Nov 1807 Recorded in: Shotts, Lanarkshire, Scotland
Father: James SNEDDAN
Mother: Rachel BURT
Source: FHL Film 1066605 Dates: 1801 - 1825
SNEDDAN, Ann Christening
Gender: Female
Birth Date: 25 Apr 1809
Christening Date: 14 May 1809 Recorded in: Shotts, Lanarkshire, Scotland
Father: James SNEDDAN
Mother: Rachel BURT
Source: FHL Film 1066605 Dates: 1808 – 1809
SNEDDON, Helen Christening
Gender: Female
Birth Date: 21 May 1811
Christening Date: 26 May 1811 Recorded in: Rutherglen, Lanarkshire, Scotland
Father: James SNEDDON
Mother: Rachel BURT
Source: FHL Film 1066604 Dates: 1698 – 1855
SNEDDON, Janet Christening
Gender: Female
Birth Date: 26 Nov 1813
Christening Date: 28 Nov 1813 Recorded in: Rutherglen, Lanarkshire, Scotland
Father: James SNEDDON
Mother: Rachel BURT
Source: FHL Film 1066604 Dates: 1698 – 1855
SNADAN, Aminta Christening
Gender: Female
Birth Date: 5 Jun 1817
Christening Date: 22 Jun 1817 Recorded in: Dunfermline, Fifeshire, Scotland
Father: James SNADAN
Mother: Rachel BURT
Source: FHL Film 1040104 Dates: 1797 – 1820
SNEDDON, David Christening
Gender: Male
Birth Date: 7 Sep 1818
Christening Date: 13 Sep 1818 Recorded in: Rutherglen, Lanarkshire, Scotland
Father: James SNEDDON
Mother: Rachel BURT
Source: FHL Film 1066604 Dates: 1698 – 1855
SNEDDON, James Christening
Gender: Male
Birth Date: 25 Feb 1823
Christening Date: 16 Mar 1823 Recorded in: Rutherglen, Lanarkshire, Scotland
Father: James SNEDDON
Mother: Rachel BURKE
Source: FHL Film 1066604 Dates: 1698 – 1855
SNADDON, Elizabeth Christening
Gender: Female
Birth Date: 3 Nov 1827
Christening Date: 30 Dec 1827 Recorded in: Polmont, Stirlingshire, Scotland
Father: James SNADDON
Mother: Rachel BURT
Source: FHL Film 1041955 Dates: 1732 - 1855
SNADDON, James Christening
Gender: Male
Birth Date: 13 Jul 1830
Christening Date: 8 Aug 1830 Recorded in: Old Monkland, Lanarkshire, Scotland
Father: James SNADDON
Mother: Rachael BURT
Source: FHL Film 1066602 Dates: 1700 – 1856
Regards,
Anne H