It is usually alcohol that has parents worried when they allow their teenage children to go to parties on a weekend. Now knives have become a common threat. In Melbourne on Friday night, a 15-year-old boy ended up in hospital with stab wounds when a party descended into a violent brawl. And last weekend, seven people were hurt when gatecrashers armed with knives and machetes barged into a birthday party at a soccer club. Officially, police say there is no evidence the problem is getting worse, but that is a line contradicted by youth workers and officers on the beat....