Key management is deemed as the fundamental essential part of any secure communication. A secure sensor network communication protocol relies on the substantial secure, robust and efficient key management system. We put forward a new hybrid group key management scheme for hierarchical self organizing wireless sensor network architecture. By using this approach, multi-level security can be achieved to secure groups of sensors at different levels. We place the cryptographic burden where the resources are less constrained, at the forwarding nodes and the access points. In our proposed scheme, access points and forwarding nodes initially perform a key agreement protocol and each sensor node in a cluster later on establishes a group key with the forwarding node using a key transport scheme dynamically after deployment.