That would absolutely never happen "... of their own accord". Children don't have sex drives. A child's view of sexuality is almost comical in its simplicity - there's simply no way that a child would understand something like sexual attraction, for example, even if it could be explained to them what it is. Your statement that intercourse is not harmful to children is downright dangerous and worrying, and I think the mods should probably remove it.
Furthermore, your argument above that dogs somehow plan to have sex with each other (and their owners) and don't have "innocence" is off the wall. It has not been generally proven that animals in any way think in the way that humans do - rather they are instinctive and fuck whenever or wherever they can. As for animal "innocence", there's not a philosopher alive who's ever gone near that one.