Annanan
Sex with a child under 16 is illegal. No discussion. You’d be complicit in rape.
No, you wouldn't. If you have provided sleeping arrangements, you will no more complicit in rape than allowing them to go out and about together where they may get it on in a field!
We had this situation with our son. I spoke to the father and pointed out that I could provide separate sleeping arrangements on separate floors but I would not promise to lie awake all night to police the arrangement. I did have a dog sleeping between them whose nails would click on the laminate floor as he greeted anybody wandering about which would wake me up though.
However, the main conversation I had with my son was about the issue of trust and the responsibilities he had to ensure that he didn't break that. I'm not naive, I know how strong the drive is for hormonal teenagers but I thought it far more important to talk to him about the implications of an unwanted pregnancy for both of them. They were both young people with ambitions for University and good careers so I hoped that they would be careful and my husband had already put a jar of condoms in my son's bathroom to ensure that he always had access to them. I know some would consider this encouraging him to have sex but we wanted to ensure that he was not only safe from unprotected sex but also from any nasty STI's going around.