John could potentially belong to either family and no, it was not unusual for a child to be baptised within months of a marriage. In fact I have seen baptisms occurring just days after a marriage and on one or two occasions on the SAME DAY as the marriage! And there is 11 months between the baptisms of John and Ann so they could both belong to the same family.
Now, one unknown factor with baptisms is that we usually do not know how soon after birth each baptism might have taken place. Sometimes it can be within days or a couple of weeks but then it can also be several years after birth before a child might be baptised.
But in many families you can tell at a glance whether or not children are baptised are likely to have been baptised within about a couple of months - and that does look to be the case with both Charles Matthews families.
Along with the above we also have some clues that help link some children to one family or the other.
Anne named a daughter Margery which is a name that does not seem to appear in her husband's Nicholls family therefore it is most likely she was named for Margery Nicholls. That locks in 13th October 1805 to Charles and Margery.
Lydia was the name of Jennifer Pollard's mother and is a name not appearing in the Nicholls family or in any of the Matthews families prior to this so that locks in the three Lydia's to the Charles and Jennifer side.
Here is a table showing those that can be locked in at this point:-
5th July 1801 - Jennifer = POLLARD
30th July 1802 - Francis = POLLARD
10th December 1804 - John = ?
13th October 1805 - Ann = NICHOLLS
3rd August 1806 - Lydia = POLLARD
27th September 1807 - William =
21st August 1808 - Charles =
19th June 1809 - Charles =
14th April 1811 - Charles = NICHOLLS
29th September 1811 - Lydia = POLLARD
I believe there is later evidence supporting Charles of 1811 belonging to Charles and Margery so one of the two earlier Charles has to also belong in this family given the 1810 burial.
The term 'infant' in those days, as I understand it, generally could apply to any child up to 7 years old so either one of the Charles baptised in 1808 and 1809 might be the one buried in 1810.
But my initial reaction is to apply the term 'infant' to the younger of the two which, in this case, would be the child baptised in 1809. Notwithstanding, of course, the fact that you yourself applied that burial to the child baptised in 1809.
And now I must refer back to another part of your comment:-
The 1809-1810 Charles cannot belong to Jennifer because that would leave Charles and Margery with two living sons named Charles!
(There are known examples of that happening but those I have seen were much earlier.) Which ever child (1808 or 1809) was buried in 1810 is the child of Charles and Margery.
And that should leave us with John (1804) and William (1807). And whilst it was not unusual, as I discussed above, for a child to be baptised soon after a marriage I am more comfortable with leaving John to Charles and Jennifer if for no better reason than without John there would be a gap between 1802 and 1806 in the family of Charles and Jennifer.
A similar argument might applied to William because if he were a child of Charles and Jennifer then the other family would have a gap between 1805 and, potentially, 1809.
So for me, unless other evidence is found to change my mind, I would have John with the Pollard side and William with the Nicholls side.
Looking at the possible baptisms for the two Charles Matthews it should not be too difficult to determine which is which ................... I theeeeenk!!
And let's start off with - where goes son John goes Charles son of John 1784 and where goes son William goes Charles son of William 1781.
Hmmm - If that were the case then it would suggest son John belongs to the Nicholls family and William to the Pollard family thus having both Charles Matthews age about 20 at their respective marriages.
But there is one further potential clue ..... FRANCIS.
Unfortunately the only close Francis Matthews was baptised at Madron in 1760 to a William Matthews which would appear to be from a generation earlier than the father of either of our Charles.
After all of that I am still not totally convinced either way. I had not actually looked at the baptisms for the two Charles prior to this and had not realised we had choices of 1781 and 1784. But if son John went to the Nicholls marriage and William to the Pollard marriage it certainly would give us both Charles marrying at age about 20. But it may also mean that it may have been the 1808 Charles buried in 1810 which would have the son of Charles and Jennife baptised in 1809.
Have you checked for Wills for these Charles Matthews by any chance? If either of them did leave a Will it could offer some hope of solving the problem.
CT
But in many families you can tell at a glance whether or not children are baptised are likely to have been baptised within about a couple of months - and that does look to be the case with both Charles Matthews families.
And now I must refer back to another part of your comment:-