Hello Heidi....I am sorry for the loss of your husband and your children's father.
I have been trying to think of what to say to your questions on explaining more to your children.
My son 39 died in 2009 from an accidental overdose of perscription drugs. He had 3 teenage daughters he left and a huge loving family. He was separated from his wife at the time so our side of the family does not know how she has explained the situation to the girls. There is such a stigma with drugs and overdosing that it breaks my heart your children and my grand children have to even think about this situation. I think (and this is just my personal opinion) that maybe they still are too young to have a complete explanation of their fathers situation. They are just coming into their teenage years and need all the stability in their lives. Most people who overdose or accidentally OD are all additcted in some way to these drugs, either legal or illlega. They have a sickness just like other illnesses that have to be recognized and treated so many times they don't mean to do what they do as the cravings and feelings they get from these drugs are stronger than any of us can recognize.
Maybe before you do explain the complete situation talk to a qualified counsler or clergy and see what an experienced counsler recommends. They are at such a tender age.
I wish that all of us could still believe he died in his sleep of natural causes instead of the truth of the autopsy.
My thoughts are with you and hugs coming your way.