I imagine that if one is, by definition, incapable of giving legally-valid consent to sex, then one is, by definition, incapable of forming legally-recognizable culpable intent to have sex. Thus, neither child could be charged with a crime, if intent has any meaning in the process. I'm not fond of criminal laws that dispense with the need that the “criminal” actually have some sort of culpable intent.
So, to me, the obvious solution is that neither party could be held criminally liable for statutory rape (or its equivalent), as neither had the capacity to form a legally-valid intention to have sex.
If force had been involved, if one party had forced the other, one could charge the offending party with some sort of assault, since there is nothing in the law that suggests that minors can't form the intent to commit acts of violence.
Just my two cents.
sitetest
That’s reasonable. As others have suggested, maybe a better legal approach would have been to charge the parents with neglect, as if the children had committed vandalism or something like that. There should be some accountability, in aid of motivating people to use better judgment!