It would not be "where near" but "near where mummy works" or "near where mummy's company is located" or "near mummy's company".
where heads a clause that requires subject and verb.
The guide took them where the fish were plentiful.
I left the car where mummy works.
The restaurant is near where mummy's company is (located).
The restaurant is near {a place}.
The restaurant is near {mummy's office}.
The restaurant is near {where mummy works}.
There are times when it may appear as though the where-clause lacks subject and verb, but these cases can be understood as a form of ellipsis:
The guide told them that bow-fishing was possible, but only where permissible.
There, it is as though the words [it is] have been dropped: "where it is permissible".
"where permissible" is actually a clause with an implied subject and verb.