An interesting question. The protagonist is fairly straightforward; it is Digory Kirke, the magician's nephew who gives the novel its title. It's his core adventure—he whose mother is ill, he who has to face great temptation with the fruit that could heal her, and so on. He's the character who the story focuses on, and who goes through the greatest growth.
As far as antagonist, I'd argue that there is more than one. I'd say that Uncle Andrew and Jadis are both antagonists at times; that's who Andrew must contest with. Polly goes back and forth between being helper, foil, and antagonist.
Antagonist: Uncle Andrew and Jadis