Yeah, obviously their behavior is morally reprehensible, especially standing around videotaping it without making any effort to contact police, even after the fact...
But to call it murder or manslaughter completely ignores what those charges entail. "Negligent Homicide", means that *your* negligence resulted in the foreseeable death of another person... *NOT* another person deciding to drive drunk, and then crashing their car into a tree near you while you watched them die.
The second consideration here, is that "mandating" lifesaving measures endangers the populace the laws should protect as well. I'm certainly not defending these kids, but its not uncommon for amateurs to try to save a drowning victim to end up drowning themselves. There was a mom a few years back that dove in to save her child after they didn't surface immediately, the child ended up not needing help... but the mom didn't surface and drowned. Its not out of the realm of possibility for someone who didn't have the foresight to avoid drowning in a pond to be a danger to anyone that might want to save them.
There *should* be some sort of obstruction of justice charge though, its a shame if Florida really allows you to witness a death and not even report it. I mean... you're legally obligated to report most fender-benders even if there's no injuries... why shouldn't people be expected to report a death ?