A boy in cheder who has no internet at home, no tv, etc, is not exposed to human reproduction in the way that others are. It's therefore up to the parents to explain to him how children are born. Should this be an automatic process, where the child learns something in gemara and asks? Has anyone ever heard of any guidance on the matter given by Gedolim/Rabbonim?
When teaching Kodesh at Yeshivas Toras Emes (High School) in Johannesburg almost 30 years ago, the Menahel HaRav Gedalia Sternstein שליט"א told me that the Rov - Rabbi Salzer זצ"ל - told him that it's best if the students learn about these things from the Gemora - BiKedusha uVeTaharo.
As relevant topics came up, we were to explain them is as much detail as needed.
I recall having to explain to a class of Bar Mitzva aged students about Nidda and counting 7 clean days and then going to the Mikva. As a newbie teacher I had no idea how to proceed. I warned them that I was only going to explain this once and that they had better pay attention. The boys paid attention, asked some relevant questions, and didn't seem bothered by the subject.
(Disclaimer: That was the only year I ever taught, as they were desperate for teachers; I work in Hi-Tech, usually.)