Maybe in the fanfiction, the Quantum Kids are from a parallel dimension where the Miraculous powers are based on quantum physics. When they meet Ladybug and Cat Noir, they initially cause some chaos as their powers interact in unexpected ways. But eventually, they learn to combine their powers to defeat an ancient evil that threatens both their worlds. For example, the Quantum Kids could help Ladybug and Cat Noir access a different plane of existence to retrieve a powerful Miraculous that was lost there.
Well, it could be that the Quantum Kids are on a quest to find a special quantum artifact, and their search leads them to the world of Miraculous Ladybug. They cross paths with Ladybug and her friends. The kids discover that the Miraculous themselves have a connection to quantum energy. Together, they must figure out how to use this connection to stop a villain who is trying to harness the power of both the Miraculous and quantum energy for his own evil purposes. The story could involve a lot of action - like battles in Parisian streets, as well as some heart - warming moments as the different characters from two different concepts learn to trust and rely on each other.