First off, my condolences to the family on their loss.
Second of all, as much as I dislike the notion of such pranks, if they have a safety disclaimer, then by all means, they are free to post it. What the viewer does in regards to disclaimed content is on them. (Hence the oft seen phrase "DON'T TRY THIS AT HOME." and so on..) Your standard consumer/viewer is expected to act in a normal manner in concerns to the product/content presented. Creators, however, should, and usually do, keep in mind how their product/content will influence their consumers/viewers. Now, if the content did not present such a disclaimer, then the creator may (and probably will) have some culpability. That being said, considering the circumstances, it would be unwise to assign full or major culpability to the content creator, to the child, or to the parents.
Thirdly, parents, despite their best intentions, should (reasonably) monitor what their children are exposed to on the internet (or rather, in media in general) and educate and raise them to discern the potential risks and consequences of an action influenced by such. (e.g. replicating a dangerous stunt vs safely building a nifty birdhouse.)
(Not saying the boy's parents were not doing so, just stating this in general.)
Finally, I find that this prank is, in my opinion, not very funny, and again the fact that it's absolutely dangerous does not make it any less tasteless. It is sad that a child accidentally ended his life through replicating a dangerous (and insensitive) stunt for the sake of amusement. I must applaud how the mother is handling the aftermath of this tragedy, and again, my condolences on her family's loss.