Dp could be Napoleon Dynamite from the film - he'd just need to wear geeky clothes and say "GAWD" a lot - or a Proclaimer (not saying DP is geeky or having a pop cos he's ginger I'm just thinking of flame haired characters).
DD could be Hermione?
You could go as a hippy chick, Meg White from the White Stripes (just need to wear red, white & black), Pippi Longstocking?
Or you could all go as 3 quarters of Abba?