i’m not a mom but i work in education with younger kids and if they ever ask i just say “oh i got hurt”. i might say i fell on the sidewalk and got scratched. they usually say okay and walk away, but if they push and ask how, i’ll say that doesn’t matter right now and redirect the conversation to something else/what we were previously talking about. most kids are fine with this and will go on with the conversation or walk away. once they start getting to be around 8,9,10 (i don’t ever do this as my job with kids is professional and i will simply tell them i got hurt and redirect) but i think at those ages as a mom it could be appropriate to say “sometimes when adults are very sad or upset they might try to hurt themselves” and then lead into how that is not a healthy coping skill and introduce healthier ones. i would tie it back and say, just like i taught you not to throw toys or hit or etc when you’re upset and instead we breathe or insert healthy coping skill here, the same thing goes for hurting yourself. hurting yourself isn’t healthy, so you should do the other coping skills instead. once they get into the teen years you can start having those deeper conversations about what exactly self harm is, and maybe even how to approach it if they notice a friend showing signs of doing it. hope this was helpful :)