The chapter is called "The World's Greatest Dinosaur War Ever" because in it, Kenny tells how he got cheated out of virtually his whole toy dinosaur collection when his conniving friend L.J. suggests that they play a game by that name. 

Kenny has a bunch of toy dinosaurs, and he likes to play games with them with L.J., but every time he plays with L.J., Kenny finds some of his dinosaurs missing.  At first, L.J. would steal just one or two at a time, but then he thinks of a way to get all of Kenny's dinosaurs from him.  L.J. suggests that he and Kenny have "one great big battle" between their dinosaurs, which Kenny, who has a tendency to speak in superlatives, excitedly calls "The World's Greatest Dinosaur War Ever".  Since it is going to be such a big battle, L.J. cons Kenny into sneaking up to his room to bring out his entire dinosaur collection, since Momma, who is aware that L.J. is not trustworthy, will not let Kenny take all his dinosaurs out at once when he is playing with the boy.  Kenny manages to get his entire collection out past Momma without her knowing it, and the boys commence pretending to destroy the dinosaurs with weapons of mass destruction.  L.J. gets Kenny to bury all the "dead...radioactive" dinosaurs in "one giant hole", and since the carnage is so widespread, most of Kenny's collection ends up underground.  When it is time to go home, Kenny forgets about the dinosaurs that are buried, and by the time he remembers and goes back to get them, L.J. has already dug them up and taken them.  There is nothing Kenny can do to prove L.J.'s duplicity, and he derides himself for his stupidity and gullibility.  He never plays with L.J. again (Chapter 3).