The ending of Les diaboliques is ambiguous, and people have interpreted it in different ways. However, some of these interpretations fit better with the mood of the movie and suggestions it provides than others. I will suggest that the best explanation is that Christina does not feign her death: she really does die, as indeed she must to atone for willingness to commit a murder.
This raises the question of the child's story. Again, there are multiple possibilities; the simplest being the one adopted by the adults in the film: he lied about seeing Christina. But it is perhaps better, thematically, to see Christina as having been a persecuted soul who, after her physical death, continues on spiritually. One blog entry I encountered puts it like this:
There is no reason to think that Christina is physically alive at the end of the movie – that would merely be a stay of execution for her, and nothing in the film implies that she stages her death as Michel had staged his. Instead, the story of her reappearance and kindness to the boy convey an idea of Christina’s soul being preserved; she is free now from her own ruined body and her antagonists’ cruelty. For a religious martyr, that’s as good an ending as one is going to get.
A very thoughtful discussion of the meaning of the film can be found on Dennis Grunes' blog; he also believes that Christina dies. On the boy, he says
one must note at least in passing the symbolic weight of Christina’s name; sacrificial and redemptive for Michel and Nicole, Christina stores a bit of magic for the schoolboy whose vision of her “risen” is a deeply touching chord on which to close the film.