I am failing to find either Margaret McKinnon or the Mason family in the 1901 census. (though others may have better luck)
I looked at electoral registers for Gateshead (as you have access to Find My Past you should be able to do the same). The death notice in 1900 for John McKinnon said his widow lived at 18 Saltwell Road.
1899
McKinnon, John, 18 Saltwell road, house (successive) 40 Westminster street to 18 Saltwell road
Mason, John Wales, 16 Saltwell road, house (successive) 159 Westminster terrace to 16 Saltwell road
1900
Mason, Edward James 16 Saltwell road, shop, (20 Saltwell Road qualifying property)
McKinnon, John 18 Saltwell road, house,
Mason, John Wales, 16 Saltwell road, house
1902
Mason, John Wales 3 Elysium lane, house (succesive) 16 Saltwell Road to 3 Elysium Lane
Mason, Edward James. 3 Elysium lane, shop (20 Saltwell road, qualifying property)
So they were living next door to Margaret's parents when the 1899 and 1900 registers were compiled.
and the Masons were on the move again (though not far) by 1902.
I've tried an address search on the 1901 census and neither Margaret McKinnon and her children, nor the Mason family are listed at either Saltwell Road or Elysium Lane.
Its reasonably plausible that newly widowed, with young children, she had moved back in with her parents and somehow or other this entire family was missed from the census.
Boo
