One of the public trees on ancestry has this as the 1841 census entry for your Dobsons:
1841 census on ancestry
Payn St, St Mary Islington West
William Dobson 45(?) Shoemaker Y
Mary Ann Do 32 Do Binder Y
Mary Ann Do 2 Y
And they have this as your Mary Ann jr in 1851, but of course we would need to try to verify this somehow:
1851 census on ancestry
8 Pulteney Street, Islington West
Thomas Goose Head Mar 43 Engineer & Printer Middx St George Bloomsbury
Lydia Do Wife Mar 45 Middx St Geo Han Sq
Ann Do Daur 12 Middx Islington
Thomas Do Son 6 Middx Islington
Jane Do Daur 3 Middx Islington
Mary Ann Dobson Visitor 11 Middx Islington