Sometimes it's more a matter of letting them ask. My oldest is 12. I actually have an attempt scar on my wrist from almost 25years ago. It hasn't come up, whatsoever with my kids, yet. The topic has arisen due to circumstances in the community as a whole. Perhaps we worry more about what to say, when it becomes a near moot issue.
Swim shorts work well, I wear due to my own self consciousness over my own body, en vogue, so to speak.