I have read this thread on and off, and the thread that triggered it, and find it very painful and difficult to engage due to my own personal experiences.
Larry, you are so very very wrong that this issue should be kept within the family.
It is NOT a parenting issue and would become the family 'secret' in a far more harmful way if it was not dealt with by outside help.
Getting professional help is not just heaping the blame on the boy at a young age. Allowing him not to understand the absolute consequence of his actions would be reprehensible and IMO this may actually be the best way of getting him back on track and having a future as a balanced young man.
Because the world won't be so forgiving if he carries any of this into adult hood. This might be his best chance to sort his head,
And the girls NEED to see that a big fat line was drawn, that NO ONE COROSSES. NOT EVEN THE PEOPLE THAT LOVE YOU, in fact ESPECIALLY the people that love you.
As a parent, making the decision to report your child to the authorities is pure hell. And I know because it is a decision I once made, not for Sexual reasons, but whether or not anyone understands this, to try and protect my son, who was 10 at the time, from what he was becoming capable of.
He understands why, and we talk and his progress from where we were is outstanding, and I am proud of him. I may have done what was best for him in the long run, but if you think for a minute it ever sits comfortably with me that I made that choice, you would be a long way from the truth.
But at 12 1/2 now, he clearly knows sexual boundaries, and as someone who regularly hosts groups of generally male teens, would express that at this age, there would have to be a serious lack of sexual understanding not to realise that this was a line that should never have been crossed.
My 7 yr old daughter knows about what is private and wrong.
So as a victim, I sadly applaud the line of action that that particular OP took. And ache for her also.
But especially for her son, because intervention now, as I mentioned before, might be course of action that resolves whatever is going on in his head to put him in this place.
And the girls, 
well maybe this mother's course of action might help them in processing what happened so that down the line they are not as deeply haunted by it as someone who represses, buries and goes on to have a troubled life because it was kept 'hush hush' from the authorities. And maybe there will be two young women one day who are secure in the knowledge that they have the right to say NO in any circumstances.
Loving and protecting your children means sometimes taking a course of action that feels and tastes wrong.
Living with your first sexual experience being at such a young age scars you in way that goes beyond words. Because who we are sexually is quite a large part of who we are as people. So if your first sexual experience comes at 7/8 and is shrouded with secrecy, guilt and shame, that is generally what you carry with you.