The angel short story is probably "Angelica" by the well-known children's author Jane Yolen.
The story, set in Linz, Austria, 1898, begins with a young boy named Addie, unable to sleep so he takes a walk and slips down a hill and into a river. He is pulled to safety by an Angel-like figure named Angelica, although she explains that is not her real name. She says he can not pronounce her real name: Pistias Sophia, or the angel of wisdom and faith. They converse and then she leads him back to his house- the Hitler home...
I haven't found matching entries for the other two, but "The Wonderful Day" by Robert Day does have a child making wishes on a unicorn horn. Neither share the same anthology on ISFDB, but they were in Young Monsters and Young Witches & Warlocks respectively, both Isaac Asimov anthologies. As noted by Daphne B. below, Young Witches & Warlocks also contains a story named "Stevie and the Dark" by Zenna Henderson which matches with your second story, including a line describing the popsicle stick cross:
He looked down at it--his precious pocket piece--two pieces of popsicle stick tied together so they looked a little bit like an airplane, and on the top, lopsided and scraggly, the magic letters INRI.
I checked out Young Witches & Warlocks from the library and it is a definite match. To add to your details, "The Wonderful Day" has the 24 hours of wishes affect the people in the town based on a feverish child misunderstanding figures of speech so that, e.g., a woman in town becomes literally two-faced, another has a tongue that wags both ways (with the inside half chiding her for her gossiping), and another man in town prone to wishing gets a horse in his yard every time her utters a wish (c.f. "If wishes were horses"). I read through 100 Great Fantasy Short Short Stories and it includes "Angelica", but none of the other stories match what you're looking for.
I think that you really are looking for two books, Young Monsters and Young Witches & Warlocks.