We're so quick as a society these days to call things perverted and jump to the worst case scenario for fear of the consequences of not doing so. I would personally apologise for putting all these sanctions in places and making the boy feel like he's "bad" and just supervise him a little more but subtlety. Oh, and keep talking to him and make sure he knows you still love him.
I agree.
Look, it's obvious you care about your children and that's great, but I feel very concerned that what you're doing is traumatizing and labeling your children.
I can't say if what the 12 year old did was a sign of something darker, what I can say for sure is that it might well not be, and that you may well turn it into something dark if you label him as that.
Children 12 years old DO NOT know what they are doing, that's why we as adults have a duty to protect them from others and from themselves.
Talk to all the children involved, but please try not to overreact. Taking them to a therapist when they haven't been to one before is a sure way to make this into a long term issue. Obviously if there were a problem you can't just ignore it, but this is far too benign to be concerned of that in an otherwise healthy 12 year old boy.
Odds are if you make a label out of this, get the 12 year old to believe he's bad and what he's done is irredeemable, the way I see it there's only 3 things that could happen. One is he really does have a problem which seems unlikely if this is the first issue of its kind, and if that's the case labeling him will have changed nothing. Two is he doesn't have a problem and now will believe he does and will suffer very low self esteem that will follow him for a long time. And three is he doesn't have a problem and now believes he does and will act out that role he's been given.
Regardless, labeling does no good.
Talk to him, tell him you're sorry if you scared him and that you're just trying to look after him and his siblings. Reassure him he can tell you anything, and ask what he's thinking, why he thinks he did it, if he's ever been involved in anything else like that.
Just, please don't set him down a path where he believes he's broken and bad and can never be any better.
EDIT: I just read the post that showed up while I was writing this one. I looked back over my post and you're right, I didn't say enough about the 5 year old. I guess because I know that as the victim, he will get the support he needs and deserves. I'm worried that the 12 year old didn't know any better and that when he grows older and does that he will punish himself all the more as a result of your reaction.
12 is far too young to carry that kind of burden.
Ask the 5 year old boy how he feels, which I'm sure you've done, but try not to turn this into any more traumatizing than it already is.
Minimizing trauma is something we all widely recognize now as harmful, which it is. But maximizing it can be damaging too.
Keep an eye on the situation, but personally, I don't think 12 is old enough to be held responsible for this. Tell him he was wrong to do that certainly, and make sure he appreciates that, but don't traumatize all of them by dragging them all over the place to a head doctor.