I used to cut my arm when I was younger. I haven't done it in decades but the scars are very much visible and they aren't easily explained away. They look exactly like what they are.
I have young kids. The oldest is 7 and hasn't really asked me what they are yet but I know they will at some point probably sooner or later. They have heard me tell other adults though that they are old scars from an accident (they don't look like any accident could have caused them but it's enough to get adults to stop asking usually).
I really don't know what to say if they ask. I don't want to lie. I think when they are older and perhaps know that self harm is a thing I can tell them the truth and explain it to them but for now I don't even want them to think that this is a legitimate way of dealing with your issues (or avoiding dealing with your issues...). I'd hate to lie though and also, they don't look like they are likely to have been caused by an accident so there isn't even a great lie that would convince them. I could just say it's none of their business or I'll talk to them when they are older but I think that would make them just more curious or perhaps freak them out.
What would you say?