My 'anecdotal ramblings' as you see fit to call them, are no less apropos than yours on the subject of playing doctor. Thank you for sharing. Thanks also for the reassurance that my opinion is as valid as any other. It hadn't actually occurred to me that it wasn't, but I am touched all the same.
"Children of this age copy, it is perfectly possible that one of the children has witnessed such an act, could have been by accident, doesn't "prove" abuse. If that is all it is, then handled sensibly and without hysteria it is not likely to cause lasting emotional scarring as what was seen (if indeed it was seen)would have been beyond the child's comprehension at the time.
The only way to find out what is really going on here is by having trained professionals investigate. (Actually a similar question has come up just recently on another thread).
"However, a great deal of emotional scarring could result in an investigation by SS. Inquiries of this kind can and have, rip/ped families apart. At best they will be left with a documented history on SS records. That's a pretty serious consequence imo"
So you don't like social services, you think social services involvement might be worse than the possible sexualisation of a child (the girl)-- this is where we part company in terms of advice to the OP. It is never advisable to proceed with questioning of a child yourself as this can muddy the waters for the professional investigation. You can't really make an attempt to be surer of your ground before making a call about the incident and your concerns.
"What I am appalled by is the tendency for some posters to "tell" the OP what she "must" do AND imply that if she doesn't she will somehow be implicit in allowing a crime to be perpetuated. This is beyond belief. We can all give our opinions and our reasons for holding those opinions. We can all say what we would do under the circumstances. I think any reasonable person should draw the line at telling the OP what her course of action must be. The OP is the one who knows the people involved, but, most importantly, she is the one who has to "carry the can" if her course of action turns out to be the wrong one."
This is possibly one of the silliest paragraphs I have ever read on an advice forum. I have highlighted the part you have simply invented.
When it comes to issues relating to sexual abuse of children, you are talking through your hat. ("A quick trawl through the relevant studies"???)
the "usual" reactions are either withdrawal, and these were not the actions of a child lacking in confidence, or aggression, but these children were of comparable age/stage of development and there was no evidence of coercion. In the first place, "usual" reactions are not withdrawal (but I'm not really sure what you are driving at here, the sense of your sentence is hard to discern). You actually know no more than I do about the personality of the girl involved here. You know nothing about the incident and neither do I except what the OP has stated. You cannot possibly make any categorical statements ('these were not the actions....' 'there was no evidence of coercion...') about the incident. Plus there does not need to be any element of coercion for the behaviour of one child to be abnormal.
-- "It caused no immediate, discernible trauma, the OP was not brought running at the time of the incident, alerted by some commotion or crying child."
Your attempt to define this incident away is verging on baloney here, and it runs contrary to all anecdotal and professional studies on sexual abuse that I have ever come across. Pray share with us what 'relevant studies' came up with these particular hallmarks of incidents of inapproriate sexual behaviour among children.
-- "We have no evidence that this was a particularly prolonged part of their play, so we can't even claim obsession with sexual play."
An incident doesn't have to be prolonged, nor does there need to be any sort of ongoing obsession for behaviour with other children to be problematic, or indicative of an underlying problem in the girl's life.