I'm really sorry that happened to you, but I will say I agree with the others and don't see any malice from your parents. I could see a more justifiable feeling of betrayal if you lived in an extremely violent area where this happened all the time and everybody else didn't let their kids be unsupervised outside, but it doesn't seem so. Parents aren't going to hover over their kids every second of every day, and your dad did what he could with what he had. The boys that raped you, those guys are the problem. If there was no adequate justice given, the system is the problem as well.

If, until this memory popped up, things were decent with your parents, I'd say they're not so bad that you have to keep your grandchildren away from them. If you feel there was something that needed forgiving, that would be the best route.