The children are baprised at Rochester Saint Margaret, according to the IGI, with two daughters baptised 1 August 1819. Looking at the PRs there was a son baptised the same day as well, George.
http://www.rootschat.com/links/03a4/Having three children baptised the same day more than likely means Peter and Sarah Ann were married condiderably earlier than 1818, possibly as far back as 1814. No birthdates indicated to pinpoint this a bit more.
Peter's occupation was a Cooper.
Saint Margaret's registers are fairly well indexed and the indexes are also available on the Medway Cityark site, a bit further on than the refs to the main registers. It might pay to check the burials as well to see if there were others that you do not know about.
Ian C