Hi Lynne , it is a bit difficult, had a look around , not sure if William was a Crebbin or Crellin. I have seen a few of your postings on other sites and you know that William and Sarah were illiterate so that could explain why there could be an error. I believe Isabella Ann Crebbin and James Nelson Crellin are their children.
Took me a while to find Sarah's parents wedding as I was checking who her mother was - her BC has mother Eliza.(beth), but finally confirmed mother's name Isabella and Paul is shown on the IGI as Tayle.
The Isabella Ann Crebbin aged 6 who was buried 7/10/1827 cant be yours.
There are two deaths of William Crebbin 1840 - Braddan aged 29 and Douglas aged 31.
Could not find them on the list of IOM immigrants to Australia or on the 1850 US Census.
There is a Marriage June Qtr 1856 Ulverston of Isabella Crellin. Marriage Manchester 1853, Liverpool 1854 and West Derby 1855 of James Crellin.
Death 1869 Liverpool Sarah Crellin aged 60 and William Crellin Ulverston aged 79 in 1879.
Just a few ideas to kick around