You can start by talking about masturbation, and proceed to the actual reproduction process.
It is completely fine if your kid finds this uncomfortable, so just delay the conversation and wait for them to be ready. And when they are, parents should also be ready to explain what the kids are expecting to hear. Again, if they are shy to talk about sex with their kids, the kids will feel some sense of shyness or shame as well.
Pre-teen years is when you talk about puberty, along with physical changes, such as the growth of pubic hair for boys ages 10-16 years and fullness of breasts in girls ages 10 & up.
Also, you might want to inform your daughters about the occurrence of menstruation.
Aside from the introduction of puberty, the pre-teen years are also the best years for parents to discuss sexual abuse with their children.
Yes, the idea of sexual abuse can be frightening, but this will help your children learn how to protect not only themselves but other people as well.