I don't think this is the same Molison family: here they are in 1871 for comparison, living in Mile End Old Town, London :
Alexander Mollison abt 1817 Scotland Head Ship Rigger
Mary Ann Mollison abt 1819 Shoreditch, Middlesex, England Wife
William Mollison abt 1849 Limehouse, Middlesex, England Son
Elizabeth Mollison abt 1853 Mile End, Middlesex, England Daughter
Sarah Mollison abt 1858 Mile End, Middlesex, England Daughter
However, given Alexander's occupation, name, and the fact they live so close, I'm sure they're linked in some way. Our Alexander was born c1803, so it's probably not his son (unless one or both are lying about their age...), but it could easily be a nephew or cousin.
Rambler