Hi Natalie,
Since you don't say when these children were born, I'm taking a guess.
I have found one James Waterfield born in Tavistock:
HO107/1883/378 p 11
34 West Bridge Cottages, Tavistock
William MARTIN Head M 55 M Journeyman Mason b Tavistock Devon
Johannah MARTIN Wife M 51 F b Tavistock-Devon
James MARTIN Son U 19 M Mason bPlymouth Devon
Elizabeth MARTIN Daur 16 F Working At Copper Mine b Plymouth Devon
Prescilla MARTIN Daur 14 F House Serv b Tavistock Devon
Hannah MARTIN Daur 10 F Scholar b Tavistock Devon
James Martin WATERFIELD NrsC 9 M Scholar b Tavistock Devon
John TRUSCOTT Widr 31 M b Cordwainer b TavistockDevon
There is no record of an Eliza Waterfield in 1851 in Devon that I could find. Sorry

Nell