A little more on saxy-jags post.
The law presumes that when a girl is under 13 she is not mature enough to consent to sex (not 16). So even if a 12-year-old girl willingly has intercourse, as far as the law is concerned, she has not "consented" to it because legally she is not able to.
The implication of this is that anyone who has sex with a girl under 13 is committing "statutory rape". There is no defence to this charge - even if a boy says the girl was willing or that he thought she was older than she was, it would not matter.
Of course, there is a major difference between two 15-year-olds getting to it and an adult having sex with an underage girl or boy. But in the former, it is unlikely that the CPS will get involved.
Parents, if they are aware of their underage children having sex, could theoretically risk prosecution for aiding and abetting the unlawful intercourse. This might particularly happen if they allowed sex to take place in their house, or if they said: "Well, if you're going to do it, here's a condom."
There is a provision under the Parenting Order, whereby parents take the responsibility for their child breaking the law, although this applies mainly to anti-social behaviour, it could be applied to underage sex.
I am not aware of any lawful provision for "punishing" underage females engaging in sex, other than the above.