I think if anything makes you uncomfortable then you should raise it with social services. If its nothing then it's nothing, but if its something you could be saving this child from sexual abuse. Aside from being dirty, are there any other signs of neglect?
Could be silly boasting if the child has older siblings or has heard playground talk or as already mentioned she doesn't actually know what intercourse is and is referring to a boy kissing or cuddling her or some other innocuous behaviour or she really has been molested.
As the mother is unnapriacgsnle I would just speak to the teacher or head teacher st the school.
When I was that age we used to talk about sex all the time and there were a few who claimed to have done it from infant years onwards. Usually with other kids (real or imaginary), occasionally celebrities , I don't think any of us actually knew what intercourse was. Obviously a young child talking about sex with an adult known to them is a different thing entirely.