Maturation is a process. Some may not be ready to understand the mechanisms at work even when adult. I would suggest that a limited honesty be the best policy, one that espouses healthy values but doesn't imply that failure is unconsciounable. In other words, "They are a part of me from a long time ago. Before I became who I am today."

Telling small kids about sadness, and even depression, done in the appropriate way can be inoculating rather than anesthetizing. It requires being aware of the maturity and emotional wellbeing of the particular child. You might be surprised how much some kids understand of what depression feels like and is like. We don't give them near enough credit.

Talking with their parents if you have a concern always helps to define not just what they think would be acceptable, but your own motivs and ideals. The fact that you've put this much thought into this already proves that you understand the responsibility. Follow that vein and research what might be appropriate for spexific situations where you would be tempted to talk about your scars and have a plan prepared. A decent text on developmental psych and child psych are invaluable tools, but try also to be aware of your limits and allow professionals to provide insight and treatment where you're out of your depth.

I'm proud of you for taking the time to think through this. It shows that you've grown, and are an asset to your friends, their lives, and their children's lives.