I think it might make sense to have an age of consent where the younger party could consent to whatever kind of sex they want with whomever they want (12 seems reasonable to me as an arbitrary age, although I am aware of the flaws of such things), and then children below that age would have more restrictions based on lack of physical development for sexual penetration, dependance on parents, vulnerability, age appropriateness of sexual acts, etc. Ideally, you could gradually ease the restrictions as the child ages.

It's really hard to write this into law in a way that is reasonable for all involved, though. You might have a system where people could apply for individual exemptions, or it would be a valid defense if one could show that the child was indeed ready for it and truly consented. You would set up the system where you wouldn't have overzealous prosecutors and cases were brought based on the wishes of the parents and the kids. Unfortunately, I doubt this will ever work, though, at least not to any great extent. The closest we've ever come AFAIK was in the Netherlands between 1990-2002.