The beginning scenes make for a touching short film in and of themselves. The boy who knows that he's growing up and will soon have other things to replace his childhood play. The boy who becomes the child in grief, the young man who has to handle "so much more than he should." The person who has to go to war and face all of the horrors therein. The man who has to return from the war and approach the new battlefield of the work force to provide for his family. The person who is so tired out from all of this that he can't even take a free moment to remember what joy and happiness are; he's just so busy trying to handle responsibility.
What makes all of this so touching is that step by step montage. With something like Hook, you have the parent who values their career as if it matters more than their family. You see it often in film (Jungle 2 Jungle is coming to mind, too, for some reason--ah, 90's films). Then they get the reminder that they need to remember that it's their family that should come first. But with Christopher Robin, you know that this character does love and care for his family--it's that love and care that let him come back half sane from the blackness of war and that drives him to spend so much time on work so that he can make things better for his wife and daughter. It's so touching because we've all seen it in real life. People who work full time don't have much time to spend at home--and yet you have to work full time or even overtime to provide for a family. So the film isn't saying that Christopher was in the wrong and needed to learn a lesson; it's saying that life is hard.
As far as the fantasy world goes, basically this film made the Hundred Acre Wood into Narnia. It's a little different from what you get when you are simply looking at stories of Christopher as a boy. There, this fantasy place is his created world, in a way his practice place for the real world; it is inevitably, by its very nature, the place of childhood that he will leave when he reaches adulthood. Narnia, on the other hand, is the place that you continue to believe in even when you grow up, even if you no longer visit it as an adult. Narnia fans will remember that Peter, Edmund, and Lucy would get together with Professor Kirke, Polly, Eustace, and Jill to talk about Narnia--but Susan, once she grew up, no longer believed in Narnia and considered it simply an old childhood game, nothing of importance. So that's kind of the same thing that this film did with Christopher Robin's Hundred Acre Wood.
This wood became a place that it was important to still remember and stay in touch with. Robin doesn't just stop visiting Pooh and Piglet and the rest; he also puts away his drawings of them and discourages his daughter from looking at the pictures, instead of telling her about the fun games he used to play as a boy and letting the games live on with her. Because life took away his spark of fun, he isn't feeding that spark to his daughter.
The Hundred Acre Wood shifted from being a representation of childhood to being the element of play, the element of fun that needs to remain part of life even when you have to add necessary things like responsibility to it.