With the growing demand for instant messaging, securing group chats has become a critical concern. While cryptographic approaches are essential for this purpose, they typically require secure and efficient protocols for either group key agreement or group key distribution. Most existing solutions struggle to achieve efficient key updates and seamless integration with real-world systems. To address these limitations, we We present ChatKID, an identity-based group key distribution protocol. At the architectural level, ChatKID employs an asymmetric design to address the resource imbalance between the group owner and group members in group key distribution. At the technical level, it develops a novel polynomial-based key derivation method that drastically improves key update efficiency, while its compliance with the SM9 cryptographic standard enables easy integration with numerous systems. We provide formal analyses of correctness and security, including resilience against five common attacks (e.g., man-in-the-middle, replay), and achieve forward and backward secrecy. Subsequently, we compare ChatKID with recent protocols and demonstrate its significant advantages in both storage and communication overhead. Notably, for group members—typically resource-constrained mobile devices—ChatKID reduces overhead by one to two orders of magnitude (i.e., to$1/10\sim 1/100$) compared to others. Furthermore, we compare ChatKID with mainstream instant messaging applications and demonstrate advantages in terms of connection dependency, computational overhead, communication overhead, and storage requirements. Finally, simulation experiments indicate its superior performance in typical instant messaging scenarios. Taken together, these results suggest that ChatKID holds significant promise for advancing group key distribution in real-world instant messaging applications.