Vehicular manslaughter. What else?
And if I was the defense attorney I would argue their deaths were the result of their failure to wear a seatbelt.
In the end, A correct court decision would consider all factors regarding these deaths.
One post says those dead are solely responsible for their deaths.
The next post says that the stoned driver should be charged with manslaughter, i.e. he is responsible for the deaths.
I suggest not becoming a prosecutor.