Now is the time
It is so important to be open and honest with your kids about things that could potentially harm them. My kids know the dangers of drugs, especially heroin, because of losing their brother. I've been very open and honest with them about all of it. Today, we had to fill a scout requirement. It was about child abuse and sexual molestation. These are important dangers your kids should know about as well as the dangers of drugs. Personally, I was not uncomfortable talking to them about it. It felt very empowering to pass on the knowledge to them so in the event of an emergency, they will be equipped with survival and coping skills that can assist them. This also applies to being honest with them about drug abuse, prescription and street drugs alike. If you think your child will be traumatized by talking about child abuse, molestation or drug abuse, imagine how much more traumatized they'll be if they were to be victims of abuse/molestation. Often times, victims of abuse will turn to drugs and alcohol to ease the emotional pain that eats away at them.
If you are waiting for the "right time" to have "the talk," the time is NOW! Of course you will need to talk with them in an age appropriate manner, but letting them know what is appropriate and inappropriate behaviors from a young age is essential.
I once read an article that said you could find out so much about your kids when their mouths are full of food, and it is absolutely true! Having dinner at the table together is important family time. Some people do not have that luxury, so if you do then count your blessings.
I urge you to talk to kids about these things. It might just save their lives one day. Talking to your kids about sexual abuse:
Talking to your kids about drug addiction: