Very frustrating. You always hope for a child to be living with a relative, that's helped me out a few time
Possibly that the Sarah, wife of William Jewkes was a little younger. Appears to be on the 1841 and 1851 census, year of birth in 1851 is listed as 1803 and there's a death in 1860 that matches. Given ages that early are not always accurate.
On the baptisms, first thought would be that Sarah's was delayed for some reason but I see Sarah's brother Thomas was born just over year before so doesn't give any leeway. Other options could be Sarah and Seana were twins and Seana's baptism was delayed for some reason (though twins are sometimes remarked upon) or it's an error and Seana (or Sarah I suppose) is someone else's. Probably unproveable.