I'm glad you asked why this is considered a tale for children, Jessica, and though I can't provide you with an unequivocal answer, maybe by working through my process I can come up with a pretty good theory.
The first thing I'm drawn to with this text, as with any other, is the tone and diction of the writer. I love Marquez's imagery, and the way he uses in medias res in this story. I mean, what better first line is there than "[o]n the third day of rain they had killed so many crabs inside the house that Pelayo had to cross his drenched courtyard and throw them into the sea..."? That first paragraph is filled with imagery, but it's not exactly the pretty kind. Pelayo, Elisenda, and the other villagers live in an ugly gray squalor, a type of poverty which seems stuck in time. There is basically no mention of any sort of technology or industrialized product until Pelayo and Elisenda become rich, and even then it's the country provincial sort. As we touched on in class, this feels strangely like a fairy tale, and I tend to think of fairy tales as inhabited by children: Hansel and Gretel, arguably Cinderella, etc. You mentioned, Jessica, that you thought Marquez's tone was sarcastic, and I can't say I'd use that strong a term--maybe more of a satirical ridiculousness--but you're definitely right that he doesn't shrink from the weird, grotesque, and odd. More likely he delighted in it.
Getting back to the text, when I first read this, I found myself reading it aloud. As I said, it's definitely strange but it's a beautiful sort of strange that's fun to say aloud. And now that I think about it, I see this story as one which would be passed down through oral tradition, in front of the village children. I can't see kids understanding it any more than you can, but even if they didn't understand it, it doesn't mean they wouldn't enjoy hearing it, imagining it. I mean, how many things did we read or see as a kid, and now experience as an adult in a completely different way? For example, has anyone noticed all the double entendres and innuendos in Shrek? But I digress.
In a sense too, I think the townspeople are childlike. Their responses to the old man with wings are both classically human and childlike: they look on him first with fear and apprehension, then attempt to put him into a category that they are familiar with (hence the angel thing) with limited success, and when that fails, they resort to fear and abuse again. Take this quote for example: "They looked at him [the angel] so long and so closely that Pelayo and Elisenda very soon over came their surprise and in the end found him familiar"--but this is very soon changed after the man speaks unintelligibly to them, and the neighborhood lady "shows them their mistake." One need only spend a day in a middle school to see this phenomenon at work: children who are perceived as different are found threatening in some way to the others, and they respond with abuse to make themselves feel better. It's precisely what the townspeople do, so in some ways, perhaps their development, they're still children. Ostensibly, they've probably never left their village, where they live in waterlogged shacks, or had a good education--hell, it's clear from Elisenda's baby's fever that they can't afford a doctor.
So, in conclusion, maybe when Marquez says "a Tale for Children," he means those people who remain at a level of understanding the same as the townspeople, who need a fable/tale like this to bring forth some inner growth. But I don't think he's being derisive, either: this is a gently-teaching fable designed to encourage people to accept those who are perceived as different from themselves while reading something fun. Just as it doesn't actually matter whether or not the old man is an angel or a freakish old man with buzzard wings when it comes to him flying away in that perfect end scene, our categorization of others doesn't really matter--we just need to treat them compassionately and equally, and not allow ourselves to trip over our fears as the townspeople do.