I bought a fearsome-looking ceremonial (or not) wooden club in the Solomon Islands in 1998 which I still own. A year or so later and certainly no later than 2000, when living in Queensland Australia, my then about 7 year old son referred to it as a "bommy-knocker". I thought that he had invented the word, as others appear to have thought their children have too, so it appears most likely that "it is in widespread use amongst children" and has been for more than 10 years now (2010). Whether Joy Cowley originated the word in her 1993 book, I can't comment. It is certainly an interesting word!
Bommy Knocker is a giant spiked club, that the title character in the book "The Hungry Giant's Soup" by Joy Cowley would threaten the villagers with if they didn't make him soup. That was the first time I remember hearing it.