I would love to give you one exact climax for this story, but I honestly think there are two possible climax points.
The first possible climax is Meg finding and rescuing her father, Mr. Murry. He is the main reason that the story is taking place. Mr. Murry is missing, and Meg resolves to find and rescue him. She gets supernatural help from the Mrs. Ws, and mounts a successful extraction of her father from planet Camazotz. Yay! Initial conflict solved.
There is a problem though. During the rescue Meg's brother, Charles, was captured by IT and didn't make it off planet Camazotz. Meg returns to the planet, rescues her brother, and safely escapes. I believe that second rescue is another possible climax to the story.
If you forced me to choose, I would say that the second listed climax is the main climax. It's the point in the story where Meg must use everything that she has learned in order to face IT and successfully rescue her brother. A climax is also the highest point in a story and nothing more exciting than it should happen after. After Charles's rescue, everybody returns home for normal life, which is for sure falling action and conclusion.