There are swim pants, would those help cover the scars? There's fine line in telling them just enough info. Most kids I know would see a scar, and ask what happened, thinking there was an accident or something. Usually any answer would work and they wouldn't ask more than that because they are moving onto their next thing already. One guy I know has a stab wound on this chest and when a kid I know asked him what happened, he told them he fell on a worm. Everyone laughed and they didn't ask anymore about it. I would be curious what the mom wants to do with this. Older kids would be able to handle the depth of it, but younger ones, it would depend on their maturity. As my kids have gotten older, they are 19 and 20 now, I have added more info about my past child abuse and why I act the way I do because of PTSD. It was gradual thing. If they saw me crying at around 13, I would say I was just sad and it didn't have anything to do with them. (they do worry, even at a young age. Now if they ask, I tell them I had a hard night with flashbacks and am feeling emotional. I am always honest, but only give them enough info that they need for their age.
Good luck! You sound like such a great guy, they are lucky to have a friend like you who cares.