These books are great for kids searching for books like Percy Jackson. I'll add to this list as I find more Greek mythology chapter books. I was so glad I was able to help her find some books, some of which she had heard of and was excited about.
Beasts of Olympus series, Lucy Coats
The Other Side of the Myth series, Rebecca Fjelland Davis
Goddess Girls series, Joan Holub
Heroes in Training series, Joan Holub
Myth-O-Mania series, Kate McMullan
Pegasus series, Kate O'Hearn