For starters, there aren't any children mentioned in the story but the baby boy that is hardly mentioned, which feels significant to me. When you boil it down, all of the adults are attempting to use the old man for their own personal gain: Elisenda and Pelayo exploit the old man's presence to make a profit, the Priest tries to interpret the old man as an angel to reinforce religious beliefs, and the sick of the town physical grab the feathers off of the old man and try to cure their strange ailments. Another point you mentioned is that everyone identifies the old man as an angel. One quote you highlighted is "he reminded them that the devil had the bad habit of making use of carnival tricks in order to confuse the unwary," and this fear of the unknown that Marquez is exploring explains why the townspeople understand the old man to be an angel.
Everything about the old man is human except for his wings, for the doctor towards the end notices that the old man's wings "seemed so natural on that completely human organism that he couldn't understand why other men didn't have them too." If he is so human-like, why must everyone see him as an angel? The answer lies in the quote about the "carnival tricks" that "confuse the unwary." When presented with something unusual or just outside of our understanding, many of us react in unfavorable ways. Some of the actions that the townspeople and Elisenda take towards the old man are atrocious, but when viewing them through the lens this quote creates, they are not that shocking.
Drawing the two points together, I think Marquez is depicting how cruel humanity can be towards the unknown and how when we identify something as familiar in order to simply name it, we create expectations for it. And just like in the story, when these false expectations are not met, we have the tendency to act with cruelty rather than kindness. To answer your question, the old man is not an angel, but because we have a need to name things to understand them, he is an angel to those in the town. "A Tale for Children" is an appropriate name for the story, because it's less about teaching children a lesson and more about observing a part of humanity that these adults are exhibiting.