The FBI lists the US Homicide rate at about 3.05 per 100,000 in 2010 (Last year with Census Data) This list of Cancer facts and figures from the
ACCO website has the childhood cancer mortality rate at 2.2 per 100,000.
Every time I've ever seen people talk about childhood cancer, they've described it as rare. When you hear anti-gun folks talk about firearms violence they almost always describe it as epidemic.