The film's depiction of Christopher Robin Milne's relationship with his parents is fairly accurate. The actual C.R. never forgave his father for exploiting him (perhaps inadvertantly) to sell books. Instead, he learned to accept what had happened, and lived with it until he died in 1996. His relationship with his mother was worse; she also disapproved of the woman C.R. eventually married. After A.A. Milne died in 1956, C.R. had almost no contact with his mother until she died 15 years later. C.R.'s only child, his daughter, Claire, was born with Cerebral Palsy. She died in 2012, at age 56.
A.A. Milne was 38, and his wife, Daphne, was 30, when Christopher Robin was born. When filming started, Domnhall Gleeson was 33 and Margot Robbie was 26. Robbie is also five years older than Alex Lawther, who plays the adult Christopher Robin.
The real stuffed toys owned by Christopher Robin Milne and featured in the Winnie-the-Pooh stories. They have been on display in the New York Public Library in New York City since 1987. According to the New York Public Library's web site, the items have been on display in the Children's Center at 42d Street, in the "main branch" of the library (the Stephen A. Schwarzman Building at Fifth Avenue and 42nd Street) since early 2009.
Director Simon Curtis mentions at 0:29:31 in the director-screenwriter DVD commentary "And this enclosure, again, is accurate to the period, but now is unused at London Zoo. So you won't be surprised to hear that this bear actually is in Pasadena, California, and we had to use film magic to insert it into this scene." Black bear footage is credited to Greenscreen Animals in the on-screen ending credits.
The trivia item below may give away important plot points.
One of the titles at the end of the film says "Christopher Robin never took any money for himself from the vast income generated by the stories". Rightly so, C.R. Milne never used the money for himself. He did however spend it on his daughter, Clare, who was born with cerebral palsy. He wrote in his biography: "I had to accept it, for Clare's sake".