Katniss Everdeen - Hailee Steinfeld (True Grit)
Turns out I know her aunt. She's 14 so she'd be perfect for this and the sequels.
Peeta Melark - Chord Overstreet. Okay, he's 21 so he's probably too old, but I still think he fits the description of Peeta Perfectly.
Maybe he has a younger brother....