The mummy costume is an awesome idea for Kakashi.
I think he’s likely to dress up in something goofy and eccentric. Like a mascot sort of thing. A bulldog one. With a huge head that his face sticks out of. 
I think Iruka would do more generic or classic costumes, like a mummy or vampire. But it’d be real nice.
Or maybe he’d just dress up as Kakashi. 
Also watch Kakashi become Konoha’s new cryptid because he’s having too much fun tormenting the littles.