Youngest child in the 1851 census could be Catherine bapt 2 Feb.1848 Saint Marys, Castletown, [illeg.] daur of Ellen WATERSON and William CHRISTIAN.
The LDS film number for St. Mary's Church (Castletown, Isle of Man) which contains the official copy of the parish register of Castletown, baptisms, 1807-1859 is FHL BRITISH Film 106714.
The FHCs will usually allow you to order several films at a time.

There is another Catherine Waterson bapt 6 Aug.1845 Saint Marys, Castletown, daur of Samuel Waterson & Anne.
John bapt 21 Dec.1842 Saint Marys, Castletown, [illeg] son of Eleanor Waterson & Samuel SKELLY?
Elizth bapt 21 June 1835 Saint Marys, Castletown, [illeg] daur of Ellen Waterson & James BRIDSON?
William bapt 20 Nov.1831 Saint Marys, Castletown, [illeg] son of Ellen Waterson & Stephen GELLING?

There is an Elinor Waterson bapt 31 May 1807 Malew, daur of Henry Waterson and --- Fell in the par reg extracts on the IGI, which doesn't seem to fit with Henry's previously mentioned possible marriage to Esther KENNAGH. There are other entries for children of Henry Waterson & Elizabeth FELL, so it looks as though the 1851 census entry and Henry's burial could be a different family.
Looking at the parish registers should help sort them out.

Sue