The film's depiction of Christopher Robin Milne's relationship with his parents is fairly accurate. In real life, C.R. never actually 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 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".