Hi Helen and Abbey,
I've had a family in London who decided to change their names - not that I'm suggesting Charles has, but in a highly-populated area it can be tricky to be sure. The only way I managed to prove and develop a theory was through DNA. I'm presuming that you are descended from the couple, so if you (or the most recent descendant from them) have taken a DNA test, it is what I would suggest.
I've had a quick look and have found this possibility on the 1881 census:
Holborn Union Workhouse:
Charles McGuiness, Pauper - Inmate, 8 years old, Unknown Occupation, Unknown Birthplace
Age is slightly out though.