kids having sex

Humans are curious, inquisitive and inherently sexual creatures. The drive to reproduce and be sexual is the strongest drive we have as humans. It can manifest at an early age in various ways and to various degrees. Under the right conditions, preteen children can very well begin to experiment naturally, or perhaps mimic sexual behavior they have seen in movies or heard about from their friends. If they are particularly driven, and intelligent, they will find a way to make it work in some fashion. The problem then, is one of how best to respond when discovering sexual activity. The consensus seems to be that punishment or loud or violent objection is the LAST thing a parent or caregiver should do when confronting such natural behavior. Gently stopping the activity without shaming the child should ensure that they won't attach trauma to their future sexual desires. If they are old enough to engage in sexual activity, then they should also be old enough to engage in reasoned conversation about how ones sexual life should be conducted -- including the emotional aspects, and the need to be considerate and respectful of the other person's (child's) feelings, fears and desires, etc.

There is likely no "one size fits all" answer to this, so the parent will have to rely on their own sense of caring for the child, and an understanding of the child himself when cautioning against such behavior at such a young age. Just keep in mind that Nature has only one goal here -- the reproduction of the species at the earliest, most vigorous possible chance -- and it doesn't know or care about societal strictures or mores.