I think you will find that she was in Leicestershire, living with her son Henry

I see you have a thread about Charles in Bucks

which does not mention Mary

It does help us to find people if you tell us as much as you know about them to start with -

Hopefully you can put this family together; just for the record, this is the entry Itsme found:
1851
HO107. P. 2085 Fol 561 Pg. 13School House, Long Whatton, Leics
COX
Henry Head M 22 National Schoolmaster B. Bucks, Chalfont St Giles
Maria Wife M 24 National Schoolmistress B. Somt. Easton ?
Cecilia Jane Dau 5 mth. B. Leics. Kibworth
Mary Mother M. 66 Visitor B. Suffolk, Mildenhall
Eliza Allen House servant 15 Hse Servant B. Leics Kibworth