Hi,
have you already searched freebmd for the people you are trying to trace? I had a glance but unfortunately I couldn't find many entries.
There is an entry for the birth of a John Duncan Caldwell in dec quarter of 1899. I already saw, but it's only a guess, an entry for the birth of a Richard Duncan Caldwell in June quarter 1904 and a Duncan Caldwell in March quarter 1906. Unfortunately, you won't know who their parents are unless you order their birth certificates at the GRO (7 pounds each)...
As for Census data, I have spotted a family which might be yours, but the ages rarely match:
1891 in Kirkdale:
James, head, 49
Elizabeth, wife, 51
Isabella, dau, 18
Duncan, son, 16
Alexander, son, 14
James, son, 22 + wife Alice and a baby son
1881 in Liverpool:
James, head, 43
Elizabeth, wife, 42
Elizabeth, dau, 13
James, son, 14
Isable, dau, 10
John, son, 4
Alexander, son, 3
1871 in Liverpool St Titus
James, 28
Eliza, wife, 26
Eliza, dau, 7
James, son, 4
Isabella, dau, 1
James is a boiler maker. I wouldn't swear they're one and the same family and they are yours, though!
As far as the other info is concerned, Margaret's marriage to Mr Hallmark etc., I think you would have to search on 1837online.com - be prepared to spend many credits, though!
Sorry - I'm afraid this is not much you can work on

Tanja