Hi
The 1881 census is free to search at 
www.familysearch.orgThere are 3 David Ellis entries who were b Caenarvon in that age range but none specifically in Penmaenmawr.  However - 2 of the entries have a second christian name which your David doesn't seem to have
This is a possible entry but as mum was a widow - it doesn't help much at the moment
Well Street Nevin, Caernarvon - all b Nevin    
John JONES 73  Retired Mariner
Margaret  72  
Fanny ELLIS widow   37  daughter Dress Maker
William  9  Grandson
David  6  Grandson
John  3  Grandson 
Ref RG11    Piece 5561    Folio 62    Page 23
I'll check the 1871 for you