It's an interesting concept, you just need to develop it more. Take some time to really build your story, to establish Neville's feelings towards his parents and the state they're in and his relationship with his grandmother. Once your reader understands the character, they'll be ready to handle your plot. Take your time with the buildup of all of that... there's no way Alice - a notable Order member - would just accept all of that information right away like that. Establish her as a character, too, and let the conflict between her and Neville build up to that great release.
It's a really great idea and start for a story. Keep writing!