How do you make my female cat not afraid to do mating with a male cat?

If they are still under age (around 1 month or so), then act. Place the male back on the bed/nursing place. If they are around 6 months, you need to separate them immediately. Kittens usually reach sexual maturity around 6 months, but some happen as early as 4 months.

The best course of action is to spay the female kitten immediately, then neuter the male kitten soon after. A male kitten's hormones take several weeks to calm down even after neutering, and will still be fertile. Spaying the female first will stop any unwanted pregnancy, and also save her from the huge risk of carrying a litter when she herself is so young and small.