Have you tried tracing the Simon Holt/Mary Jackson connection to determine the ages of this couple. Whether or not Mary Jackson was a young widow capable of bearing more children or maybe she brought children into the Holt marriage from the Jackson marriage..?
The Holt:sleuth_or_spy: marriage is recorded at:
The Church of St.Mary the Virgin/Blackburn/Lancs.
Simon Holt (bachelor)(X) residing this parish
M: 17 Sept/1805 Blackburn/Lancs
to
Mary Jackson (Widow)(X) of this parish
to be found on the Lancashire online Parish Clerk Project-
Marriages in Blackburn from 1801-1809
page:323 Entry:1284 Source: LDS Film 1278807