Some 21,000 Palestinian children are considered missing in Gaza, a leading children’s charity said on Monday as Israel’s brutal offensive kills thousands of children and leaves many more orphaned. Save the Children estimates that 17,000 children in Gaza have been separated from their parents and left unaccompanied while 4,000 are believed to be dead under the rubble of the besieged territory. In eight months of war, Israel’s sea, land and air attacks have laid waste to swathes of the enclave, killed at least 37,600 people and made the majority of the 2.3 million population homeless. The UN est…