My mother read this book to me, and I have read it to all of my children. It is, quite simply, an irreplaceable element in the childhood of an English-speaking person (some of my children were born in another country, and learned English partly with Milne's help). 
 A.A. Milne turned his son's play with his stuffed toys, and the stories he told his son about them, into a book which is beloved of millions. 
 Milne wrote a poem about Christopher Robin and his nanny watching the changing of the guard at Buckingham Palace. The boy was pleased with what he knew about the King, and asked if the King knew about him. His nanny said he was "sure to" -- and today, of course, the current occupant of Buckingham Palace and her children and grandchildren and great-grandchildren certainly do know the story of Christopher Robin. 
 Let's keep this simple: If you know a child who hasn't read Winnie-the-Pooh, read it to her/him at your earliest opportunity. If they want to read it themselves, they can, but I recommend reading it to them.