1841 Wales Census HO107/752/1; Folio: 9; Page: 10

Borough of Monmouth, Parish of Monmouth
Monnow [?] Street Cock Alley 5

Mary Hughes - 45 - No
Catherine Hughes - 10 - Yes

This is the only Catharine/Catherine Hughes aged 5-15 that I could find on the England and Wales 1841 Census.

National Burial Index:

John Hughes
Burial Date : 8 May 1833
Age : 45
Recorded in : St Peter, Dixton Newton, Monmouthshire, Anglican (Parish Register)
Data Provider: Herefordshire Family History Society

Could this be your John Hughes ? It would explain why he avoided the censuses.