Have you located the births of both Eliza and Henry Robert, and how sure are you that both of them are the children of James and Sarah?
I can see what might be them in 1871 with three kids in St. James Clerkenwell.
RG 10/387 64/67
James Adams, 29, carman b. Bristol
Sarah, wife, 36, b. London
Eliza, dau, 11, b. London
Henry, son, 4, b. London
James, son, 2, b. London
(James' baptism may be the one at Saint Andrew, Holborn, 2 August 1870)
Given the gap between Eliza and Henry, and the fact that James is a fair bit younger than his wife, there's a good chance that Eliza is the child of Sarah from a previous relationship. The reasons for not finding the marriage might be:
1. Sarah was previously married, and used her married name when marrying James
2. They were not married, for whatever reason (legally couldn't since one or other was still married to previous spouse, just never got around to it, etc).
Do you have them on any other censuses? From which record did you get Sarah's maiden name, and where is she listed as b. St. Dunstans?