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.
I would suggest that the story might be difficult for children to fully grasp because it operates so much in the realm of doubt and a lack of certainty. Marquez's work offers little in terms of certainty in its plot structure. We, as the reader, end up gauging little in terms of whether the old man is an angel and whether his presence is one that relates to redemption. The lack of certainty is where the work is a challenge for children who are accustomed to a certain sense of certainty in their early comprehension of literature.
Marquez plays with absolutism in the development of his work. The result is one in which there is questioning and a sense of development that is not explicitly stated. It makes it difficult for young audiences to fully grasp the story and for them to fully understand the story's implications.
If adults are struggling with the ambiguity that the work is built upon, younger audiences might be a bit more fledgling in their grasp of the ambiguity. Avoiding the use of the word "appropriate" and what it seems to convey, I would suggest that the work provides too many challenges to the younger audience.