One of Julia Donaldson's? I love Zog (great female character) and of course The Gruffalo.
We had a fab book last month which was an alternate version of the three little pigs, where the pigs were horrid and the wolf was lovely and kind. Would be good possibly as the children already know the traditional version?
Something easy for the parents if the children need to come in wearing costumes! Julia Donaldson is a good idea, maybe not just limiting it to one book? My DS's school did a Roald Dahl theme last year which was pretty easy. DS went as Fantastic Mr Fox. He wore a suit that he already had from a family wedding with a fox tail and ears. This year it's bloody myths and legends which is far more esoteric to sort out!