I am looking for a fiction book I read as a child, probably in the 1970s, about a brother and sister living in New York City (possibly Soho?).  They are left alone and their parents do not come home, so the brother learns to scrounge food on the streets.  His sister is afraid to go outside.  The boy meets a children’s book illustrator in another building, who begins using him in her drawings.  She takes him to the beach (the Rockaways?) in her convertible, and I believe she eventually adopts the boy and girl.