Justice? No. Natural selection? Yes.
It's got nothing to do with natural selection, sorry. Not a single solitary thing; that's a really lame suggestion.
Now, the fact that
she's a child-murderer does play into natural selection, since her behavior is not conducive to passing on her genetic heritage - a tendency to kill your own prepubescent children pretty much writes you out of the Book of Life.
The
eventual execution of the woman, doubtless years or decades after she's been effectively removed from the breeding population, has a negligable limiting effect on her genetic impact on future generations. She's had her children, killed them, and due to the likely severity of any non-death sentence will almost certainly never have the opportunity to breed again. Even if she does, circumstances dictate that any offspring will be pretty disadvantaged in the competition for resources and mates.
If people get off on the idea of killing "bad people" that's one thing - but they should not throw around terminology like "natural selection" without thinking a little bit about what it means.