Diane Silvey 
Time of the Thunderbird [PDF ebook] 

Kaya and Tala, the adventurous twins, are back from their exploits in Spirit Quest on a new mission to discover why children are disappearing from one of their tribe’s villages. Earth dwarves are being blamed for the missing children, but the twins are sure they’re not at fault. Something very sinister is happening, so once again the sister and brother set out with Yahet (Y for short), their friend and companion, to rescue the kidnapped children. Along the way they meet a mysterious owl, a cedar ogre, demons galore, Aixos, the most ferocious of all sea serpents, and the Thunderbird himself!