The subtitle is "A Tale For Children," and the story is much more in the vein of older fairy tales than modern stories that are sanitized and simplified. By presenting a possibly theological and ethical problem, and not claiming any specific moral or unassailable truth, the author challenges the reader to stretch their own thinking and mind in considering the meaning of the "angel" and its place in the story. For children, this ambiguity is a good method of developing critical thinking, as the child is forced to decide what the story means instead of being told what to think.
[The priest] came out of the chicken coop and in a brief sermon warned the curious against the risks of being ingenuous. He reminded them that the devil had the bad habit of making use of carnival tricks in order to confuse the unwary. He argued that if wings were not the essential element in determining the different between a hawk and an airplane, they were even less so in the recognition of angels.
(Márquez, "A Very Old Man with Enormous Wings," salvoblue.homestead.com)
Modern assumptions of what is and is not appropriate for children are based in a mode of protection; it is assumed that children are not capable of understanding complex ideas and should be sheltered from harsher realities until their minds develop. In contrast, this story challenges the developing mind to come to decisions regarding religion and reality, and while it might be deemed inappropriate for religious, thematic, or even cultural reasons, it is no more inappropriate than much of television programming today, not to mention content available online.