In Wireless Sensor Networks (WSNs), clustering is the suitable topology to save the energy of sensor nodes. In this paper, we provide a secured cluster formation by proposing a new symmetric key management scheme for hierarchical WSNs. The new scheme is called EAHKM (Energy Aware Hierarchical Key Management in WSNs). EAHKM needs the pre-distribution of only three keys in each sensor node before deployment, and it ensures a secure cluster formation after deployment. EAHKM assures the establishment of a pairwise key between each sensor node and its cluster head, thus the establishment of a broadcast key in each cluster in the network. Simulation results show that EAHKM provides an energy-efficient, flexible and scalable solution to the key management problem in hierarchical WSNs, and it presents a good resilience to node compromising attacks than other hierarchical key management schemes.