Regulus would so be egotistical and over the top and go as Salazar Slytherin! He'd so get into the part and dress accordingly.
Theodore would go as Robin Hood, bow and all. He'd look rather dashing methinks plus I think he'd enjoy doing so.
Nanaia is going to go as Tinkerbell from Peter Pan. She'd be a bit bashful about the short dress but she'd wear tights so she didn't feel so exposed.
Natsumi would go as the Wicked Witch of the West. She's not above joining in costume dressing if teachers like Joseph will do it
Valentina will go as Red Riding Hood.
Briar will go as Catherine from Wuthering Heights.