Think about how you'd have wanted your parents to react if it was the other way around. Obviously if they are 13 I'd be a bit concerned and ground them probably but also tell them to be safe. If they are age of consent + it's their choice so just makes sure they are happy & safe
Already been there and done that. You can't keep a leash on them. I told him over and over (and to this day even) to be very careful because having a kid at his age would be detrimental to his future plans for his life.
If he asked me to buy him condoms or something, I probably would have but he never asked me or anything.