My kids are almost 4 and almost 10. I have lots of scars on my arm, and currently healing cuts from a sui attempt.
They ask, and so far I've gotten away with "just boo boos", "it's nothing", "don't worry about it", "mommy had a little accident", things like that. The older one thinks the scars are cat scratches, and that's what he tells people.

I don't know what I'm going to tell them when they get older. My older son also has a mental illness, and I am so scared that he might start to si some day. That thought just kills me.