The werewolf army, led by Fenrir Greyback, was a group of werewolves who intended to infect as many people as possible with lycanthropy in the hope that one day the werewolf community would be large enough to take over the wizarding community. The werewolves, particularly Greyback, specialised in attacking children. This way, they presumably hoped that being infected young would cause them to integrate into the werewolf society sooner.