I would argue that the three most important events, central to the plot, of Charlie and the Chocolate Factory are the announcement of the golden ticket contest, when Charlie finds a golden ticket, and when Mr. Wonka invites Charlie to inherit the factory. With these three plot points in place, much of the other minor detail and events could be changed while still maintaining the basic story arc. In fact, the film adaptations of Charlie and the Chocolate Factory made some changes to the story, but because they maintained these three major plot points, the plot itself was preserved.
The announcement of the golden ticket contest is the event which really kicks the plot into motion! Thus far, we have come to know Mr. Wonka's candy factory as a relic, just a part of the background of Charlie's life. We know that Mr. Wonka has become very secretive in order to protect his recipes, so when he decides to open up the factory to those few ticket-holders, the whole world is in a panic to find one. We can feel the excitement and tension building as one by one, the tickets are discovered. At the same time, one feels sad for Charlie, who has very little pocket money and can't afford to buy lots of candy like the other kids at his school.
As luck would have it, Charlie does come across one of the golden tickets! This draws Charlie right back into the center of the story-- he is the protagonist, after all. Through Charlie's mindfulness and kind heart, he manages to avoid the sort of terrible accidents that befall the other visitors to the factory. The other children fall victim to their own greed, pride, and glutton, but Charlie has grown up very poor and has never known the luxury of greed.
Charlie's good behavior at the factory, and the fact that he is the only unencumbered or uninjured child left, inspires Mr. Wonka to take Charlie on as his apprentice. As the climactic point of the story, we can feel the excitement and opportunity coming to Charlie. The story is resolved quite quickly, with Charlie's family coming to live at the factory.
With these three main events of the story, one could make quite a lot of changes to the other details while still preserving the essence of Charlie and the Chocolate Factory.