Kids guns have been around as long as guns have. In Appalachia many kids got their first gun at age six, almost invariably a junior .22 rifle. The average age for getting a personal rifle was 11 or 12, though. I was probably seven or eight before I started shooting a beautiful Remington 512 (a tube fed bolt action which are now hard to find, though the 511 seems common enough).
This kind of tragedy happened on occasion (Chuck Yeager accidentally killed his baby sister), but it is very uncommon. However, modern indulgent permissive parenting and giving children tools of lethal force don't mix very well.