I?ve name-changed for this as it?s a sensitive topic ? please don?t out me if you recognise me!
Definite yes to going back to CAMHS. (Also to Explosive Child for the anger issues)
I?ve had a bit of mildly similar stuff when DS was about that age, for us it was nothing to do with abuse or seeing images, just an unfortunate effect of autism-spectrum issues. Some kids with social/communication problems can be amazingly unaware of what?s appropriate behaviour and what isn?t, and unaware of whether another kid is happy or not, while kids with impulse issues may not think, they just do it because it?s interesting / feels nice.
I would tell your DS in a very clear concrete way what?s allowed and what isn?t. Remind him which bits of the body are private (basically everything that?s covered by a swimming costume!), that we do not show other people those bits, or rub them against other people, or whatever else, and that ?trousers and pants stay on?. (If he has an ASC you would need to be extra-clear about exceptions to some of the rules, such as a doctor needing to see private parts!) You should also talk to your DD, as she may be confused. And repeat the message every time you have a visitor, whether your DS likes it or not ? and my DS did not like it, he used to get very angry, but it had to be done. I wouldn?t emphasise the stuff about how naughty it is any more because that just made DS angrier and more resistant to listening ? keep it concrete, this is what we do, this is what we don?t. DS gradually got used to ?the talk?, then matured and didn?t need it. And yes, increase the supervision; if you do catch him at it then punish as for other naughty behaviour, time out and end of visit or whatever.
One thing to be aware of ? you may get an unexpected visit from social services. Not necessarily anything to worry about, but it can be a bit of a shock out of the blue! Not personal experience but it?s happened to parents of a few kids I know with ASCs, the kids says or does something inappropriate, the school or other responsible adult can?t be sure there isn?t a safeguarding issue, a social worker turns up on the doorstep, talks to the parents, and goes away (usually to close the case) If the school feel they can?t rule out the possibiiity of a safeguarding issue then they are obliged to report it and social services have to investigate and they?re not allowed to warn you. For this reason I would try to talk to CAMHS before telling anyone else (such as the Triple P advisor)