It is often difficult to distinguish between normal and healthy childhood sexual 'experimentation' and sexual deviance resulting from exposure to pornography or sexual abuse

Where very young children display sexual knowledge and behaviour beyond that which is appropriate for their age for example enacting adult sexual scenarios and in their play with other children or with their dolls and stuffed animals.
it is usually regarded as an indicator of sexual abuse

However, an increase in the sexual behaviour of children on the edge of puberty is a separate issue and possibly should come as no surprise
In our current society before a child reaches puberty, she or he has probably been exposed to thousands if not tens of thousands of sexual messages, many of which may be incomprehensible and confusing.
Even in my own childhood over 5 decades ago I remember singing quite sexually implicit songs set to popular tunes of the time passed down by older siblings

While it is true that children who have been sexually abused have a greater tendency to display sexualised behaviour it is now also understood the same can happen when children have been exposed to or confronted with adult-like sexual behaviour.

From an article by Dr. Kaeser (who teaches human sexuality at NYU and C.W. Post College) looking at normative vs. problematic sexual behaviour of children
aimed at parents and teachers - it is suggested

"Evaluating the appropriateness of children's sexual behaviors often requires an assessment of the context in which the behaviors occur. While there are sexual behaviors that in and of themselves are problematic (e.g. simulating or attempting intercourse), we frequently need to evaluate them in context. Contextual factors that may need consideration are:

Frequency and repetitiveness of the behavior
Use of force, aggression, or coercion
Age discrepancies in children involved in a behavior
Equality or mutuality among children involved
Expressed intent "

If we applied this to the case in discussion we find no use of aggression or force; the girl herself (who was only 2 years younger and known to the boys) participated in this occurrence by removing her underwear

It really is by no means 'certain' that any of these children had been sexually abused although studies have shown that when looking at abnormal sexual behaviour in pre-pubescent children the girls involved were more commonly victims of sexual abuse (The British Journal of Psychiatry)