Cress and Carswell Thorne. "Cress" by Marissa Meyer. The Lunar Chronicles.
Mackenzie Foy is my second choice for Peony. I would actually prefer seeing Mackenzie as Peony instead of Willow because she, like Willow, has a true acting career, but unlike Willow, Mackenzie represents Peony through her looks too.
Katie Chang would be perfect for Cinder. I have never seen one of her movies but she has the looks (except for the Chinese part bc in the bk, cinder is "European"/lunar but that doesn't matter) I bet she can play the part