I realized yesterday what was missing in the Percy Jackson series.
Great children's literature has a THEME.
Take Harry Potter for instance - if you asked me what HP was about, I'd say "It's a story about love, sacrifice, and death."
Same goes for Lord of the Rings, The Chronicles of Narnia, The Chronicles of Prydain, and every other great children's story - they all have a theme that brings the whole series together.
If I was asked what Percy Jackson was about, I'd have to say something like, "It's about a boy who finds out he's a demi-god and in the first book...and then in the second book..."
There are certainly hints of where the books could have gone - Percy could have been about growing up, or betrayal, or family, or what have you, but there isn't a unifying theme in the books.
Not that they're bad stories. Book II is loads of fun and I adore one of the newer characters. They're really good, solid stories.
But they aren't great children's literature.