So, in short, the government of Tennessee wants children to be murdered. Because that's all this will cause.
Yes, we should have severe punishments for this crime and crimes similar to it. But execution for any crime just means the person committing it has no reason not to go to the extreme to cover it up. In this case, that means murdering the child after raping them, because they're already going to be killed for the first crime in the first place. They might as well make sure their risk of being caught is lessened by getting rid of the kid entirely.