I actually wouldn't say the Aethiopica is so horrible as all that. The problem is more that for us as contemporary readers we have certain expectations about what this type of story should look like -- complex characterization, reasonably plausible plot development, causal connections between events etc.
None of which are really relevant criteria for judging the ancient Greek romance as a genre (or the medieval romance either, for that matter). I've studied the development of the novel a bit, and it's really an entirely different world governed by a different set of rules and it's hard for us to adopt that perspective. There are a number of scholars who maintain the early novel developed out of the fairy tale; at any rate, some of the stylistic similarities are clear: the stereotypical characters, the role of fate and coincidence, the separation and reuniting of the lovers, the "happily ever after" moment at the end. If you think of it as a fairy tale that's trying to turn into something else but hasn't quite managed it yet, the story doesn't seem quite so bad.
As far as being interesting and accessible, another Greek romance, Daphnis and Chloe, might actually be better in this respect. The plot isn't nearly as far-fetched or complex and we do see the young couple growing up emotionally. However, there are parts of it that would probably not be appropriate for a 7 year old -- there are no explicit erotic scenes that I recall, but the story does involve the children's discovery of how men and women are different and it's fairly frank about the treatment of bodies in general.
In my third-semester Greek class we read excerpts fom Xenophon's Education of Cyrus, which included some fairly interesting stories. Might be a little old content-wise because it does deal with military expeditions and women being taken captive and such, but there may be sections which would be suitable.
IPHIGENIE: Kann uns zum Vaterland die Fremde werden?
ARKAS: Und dir ist fremd das Vaterland geworden.
IPHIGENIE: Das ist's, warum mein blutend Herz nicht heilt.
(Goethe, Iphigenie auf Tauris)