Be truthful about the situation you were in. You may want to point out some of the negative effects this had, like impaired judgment. You should also point out that the cannabis you used in the past was WAY different from the stuff being produced today – today’s THC is much more potent. Explain that you want your child(ren) to be healthy, and with all the new research on how cannabis use affects the developing brain you don’t want your child(ren) to use.