"The Girl Who Loved the Sun" is about a human girl who wants the sun to love her. It goes about as well as you'd expect, though the sun is apologetic. It doesn't help that she had started falling for a suitor that fell for her. In a nutshell, All Love Is Unrequited.
Everard's Ride has an extremely Bitter Sweet Ending. Let's just say it involves The Unfavorite elder sister going off to marry a Duke from another land rather than accept her father's unfair punishment of packing her off to boarding school.
"Aunt Bea's Day Out" has a brief moment when one of the kids loses their teddy to the military, and the whole family loses Aunt Bea. There's a bit of Adult Fear until she phones them from another country.
Both of Polly's parents abandoning her for their selfish purposes. Her mother in particular takes out her bitterness on Polly. As Chris Hill notes in his essay "I Blame It On the Parents", "There seems to be no hope of reconciliation here."