You can post almost anything. We have one restriction: Characters under 14 years old can't be sexual or sexualized. Characters under 14 can't masturbate, can't be described nude and can't participate in any explicitly described sexual manner.
If you must have those characters, then anything sexual to do with them must be described in a clinical manner. Think how they would do it on public television. So never in any sexualized manner or salacious descriptions.
In that vein, you can't have fictitious characters that are supposedly older and yet somehow behave or look like underage children. That will get you booted off the site.