Another take on this... I used to look after 6 children, the older 5 were boys.
At one point when the little girl was about 2 her next 2 older brothers (4 and 6) were very curious about sex. Presumably having heard about it in the playground, or asking where she had come from...
They found it all hilarious, and we had much talk about "sexing" people during meal times! however they had NO idea of what it really meant, or any "sexual" feelings regarding it.. all they knew was that a daddy puts his willy in a mummy.
I had them in the bath one evening when the 4 year old asked if he could "sex" his sister! I said no, and explained that we don't do that to brothers or sisters. But later that evening found the 3 of them in the bedroom playing at "sexing" each other!
In this case it was merely exploration. They wanted to know what it was all about.
So my point is, that there ARE reasons why a boy of child would know the ins and outs of sex.
A lot of children get over-enthusiastic with hugs for example.... maybe this is just taking it one step further. Children don't always know when to stop.
Of course, I am not saying that it is always this innocent. It would depend on the situation.
At the end of the day it is up to the parents involved to do something about it. If the girl is really that traumatised, and if they believe that there was some kind of sexual intent on behalf of the boy (which I personally find hard to believe)... then they should either have gone to the police or social services.