She did not want Walt Disney or Walt Disney Productions to make any more films about Mary Poppins. I thought she had put this in her will. I guess it depends on who currently owns the copyright to the books.
I'm sure that Disney legal will cover their bases when it comes to the use and likeness of Mary Poppins. Maybe it'll be something akin to the fact that the already own the rights to the "Disney" Mary Poppins, and will build from there, not necessarily from the books.