Hi Keith,
Having looked at the actual document on the 1911 census, there is a transcription error with the address. When i entered 'Cathart Street' only no 67 came up, but where John Thelwell has signed it says 67 Catchcart Street, Birkenhead - your family are living in 2 rooms and the other 67 Cathcart Street census (John Joseph) family have 4 rooms.
Do you have the census as I can pm it to you. There are two children listed on the Thelwells that have died and been scored out.
Kind regards,
Bumble