It's almost impossible to say whether or not it actually is. Although alot of what was actually done to the child mirrors things in the film (i.e The blue paint in the eye), you can't blame the murder entirely on the movie. Bare in mind that Jon and Robert's upbringing was absent of parents to teach them this sort of stuff, which is how they came across the movie in the first place.
Personally, I'd say the movie influenced them, but it didn't scream to them "Go and abduct a child and kill him".