Thanks for your reply Sue. There do seem to be two short-lived Sarahs:

Sarah d. of Hugh Kavanagh & Margaret c.5 Sep 1850 St Barnabas IGI ext
1851 census: Sarah Cabner, d, 7mo. Douglas [1850]
Cavendish, Sarah, age 3, buried Braddan 12 Jul 1853 [1850] [Lawson burials index]

Sarah d. Hugh & Margt Cavendish, Douglas, Boatman, bapt. Aug 21st 1859 St Matthew’s register
1861 census: Sarah Cavendish, d, 1, Douglas [1860]
Cavendish, Sarah, (Douglas), 2yrs, buried Braddan, 9 May 1861 [1859] from IOMFHS website Braddan 1861 burials