We encourage you to deliver the message as best you can in an age appropriate manner. This is best done by explaining addiction as being very sick. Differentiate "sickness" from what the child experiences (colds, flu) and explain that sometimes when people get very sick we have to wait very long times because we're not sue when they'll get better.
Our experience is that children almost always know more than we think they do. It's also likely that your kids have seen addiction portrayed in television, moves, and video games. Please consider this an opportunity to educate them about the dangers of substance abuse and the risk factors of being genetically predisposed to addiction.