Ok...so I've taken a look at the Donkin/Duncan/Cassidy leads.
Margaret Cassidy married Samuel Duncan in Sunderland on the 3rd March 1888
I too can find them on the 1891 census, although I cannot find a birth registration for John Duncan, aged one year at the time.
I think Margaret may have died on the 29th March 1895, aged 23 years - that death, I think, did not take place in Monkwearmouth.
Other than that - the family disappear from census records.
The Robert Donkin born in 1885 to George and Ann (late Collins nee Cassidy), I believe may have died on the 24th September 1886, age given as one year.
The family were still living in Spelter Works Road in 1891 with children, John, 21 years, Joseph, 14 years, and David, 10 years.
So, I think we can rule them out.