Having seen Jenna's link, one must look closely at the article. Firstly, the firework trade does not use the words "Industrial firework" or the phrase "Onion bomb". The item in question was most probably a firework display shell which is a category 4 firework and only for professional use. We must ask how these children got hold of it. The only two ways that I can see is that they either stole it or were sold it by an unlicensed person who imported it or stole it themselves.
Having said that, I hope that the children involved make speedy recoveries and I wish them well.