It's so important to talk to your children about sexuality starting at a young age. Even preschoolers know that boys and girls are different. Use that as an opportunity to start teaching them about respect for their body and being respectful of others.
Pay attention to the messages you are sending your child about sexuality.
It's not unheard of for girls as young as 8 to begin menstruating so I encourage you to not wait until you feel your children are old enough to understand because in reality there is no magic age at which to talk to them about sex.