Plymouth St Andrew baptisms and marriages for this period don't appear to be on the IGI, Freereg or Devon Online Parish Clerk. Baptisms seem to be available in printed form from Devon FHS for the period 1813-39, and marriages from 1813-37. The other option is that you can order and view the microfilm from your nearest LDS Family History Centre.
Have you tried looking for the children in the 1841 and 1851 censuses? You might get lucky.