Hi Patrick James,
Children under the age of consent is considered incapable and unable to consent to sexual intercourse. Therefore, in the eyes of the law, they could not have engaged in consensual intercourse.
If both children are under the age and did "consent", then neither will likely press charges and there should not be an issue. However, if the parents found out and wished to press charges, then both children may be committed a sexual offence against the Crimes Act. If one is slightly older, or the person who initiated, then they may become the perpetrator and the other victim. It really depends on the evidence and circumstances of the incident. However, it is illegal.