There is actually legal term; negligent entrustment.. in which a friend/family member CAN be determined negligent in the case of accidents, etc.. I would look at this carefully. Like, if you knew he was f'd up, let him drive your car.. he hit and killed people.. you get sued for negligent entrustment. The best example of this is the family members of the individuals killed by the drunk lady in New Jersey, are suing her brother as he knew she was an active drunk and let her borrow his car. I don't know how this applies to child care and resonsibility but I would certainly check with a lawyer before assuming you are free and clear of any responsibility when it comes to leaving your children with an active drug addict.
However, I totally agree w/ Dgillz. At the point that you'd be concerned about negligence charges in court, you might be preoccupied by much more serious and heart wrenching matters of whatever it was that happened on his (and there for your..) watch, after knowing he is actively addicted.