Its not "unspeakable", to point out the pointlessness of the fact that the child killed him or her self, but I believe that to say that the child was "irresponsible", deserving of a "darwin award", or "stupid" is in itself wholly unnecessary especially when I can count on no hands the number of people here who have a full account of the situation in which these children took heroin.
How was the drug presented to these people ("hey man, its just a little coke... you've done it before!")? Were they intoxicated to begin with? Were they really in a state in which they could have made a better decision? Hindsight is always 20/20, unless you're the dead kid. I'm sure the children would agree, that the decision they made which resulted in their death was a poor one, but they certainly aren't here now to confirm that or to reflect upon the ramifications of their decision.
Of course, we can sit here all night and point out who to blame, but the reality is that only those now-deceased people knew exactly why they did what they did as well as the circumstances surrounding that decision.
If a child dies because he was hit by a car, you hold the driver of that car responsible, right? He should have been paying attention, he shouldn't have been speeding...or you blame the city, there should be a lower speed limit, there should have been a light at that intersection, etc.
If a child kills themselves, why is it any different?
Simple. The circumstances of the former scenario are knowable (witnesses, forensic data, crash site analysis, etc.) whereas the very nature of suicide, as a personal act usually directed towards others, whether intentional or unintentional, renders the circumstances mostly unknowable and hence (I believe) immune to judgement. Suicide notes may provide insight in some cases, but we really don't have much to go on in this one (an accidental, self-inflicted death).
Finally, let me ask you; if one of these kids had made the same decision to take this drug and it nearly killed them (and they were able to hit "rock bottom"; they received a needed "wake-up call"), the result of which was a long stay in the hospital which led to months of rehab and in the end, a full recovery and the promise of a clean, productive life, would the decision still be as poor as if the person had simply died as a result? (Yes, in this case, fortune does truly favor the foolish, I suppose)
We can assume here that had the person not taken the really bad heroin, they'd no doubt still be on some pretty nasty stuff. I'm not saying that we nor the person in question should necessarily be glad that they took the bad heroin even though the end result was desirable (no ends-justify-the-means nonsense), but the judgement placed on such a decision will be decidedly different given different outcomes to the same decision.
I think we can see that the decision cannot be considered universally poor, but is considered stupid, poor, or senseless only in the event of certain outcomes (a needless, and pointless death). So, I find judgement calls proclaiming the poorness, stupidness, and senselessness of these children's decisions to be misguided and I take them with a very large grain of salt.
Sorry if that was long-winded. I've a tendency...