Of the two, I prefer the Cecile mystery to Marie-Grace's. It does start out rather slow (a lost necklace is really low-stakes) but does get into a more interesting and, I think, little-known aspect of American history (maroons, who are far more fascinating than a necklace).
There must be quite a few parents who subscribe to the belief that one shouldn't tell children "grown up" problems, even when those problems directly affect their lives. I've never personally seen any parents like this, but they must exist, considering what a constant plot point it is in lower grade...