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.