While P.L. Travers wrote 8 books about Mary Poppins it sounds as though this storyline is a new construct. It is NOT an adaptation of Travers Mary Poppins Comes Back which is another story about the Banks children as children.
What is definitely new is the character of Jack the lamplighter played by the star and creator of the hit Broadway musical phenom Hamilton, Lin-Manuel Miranda.
Rob Marshall (Memoirs of a Geisha, Chicago) who directed Blunt in Into The Woods directs from an original script by David Magee who gave us the beautiful adaptation of Life of Pi and Finding Neverland.
“I am truly humbled and honored to be asked by Disney to bring P.L. Travers’ further adventures to the screen. The iconic original film means so much to me personally, and I look forward to creating an original movie musical that can bring Mary Poppins, and her message that childlike wonder can be found in even the most challenging of times, to a whole new generation." James Marshall, director
The movie is slated to open on Christmas Day 2018 which means lot of Mary Poppins' dolls in Santa's Sleigh. I just hope Mary Poppins still has her carpet bag & umbrella.