For Anna: You can take your pick from a couple of Anna's outfits; be fancy in her green dress for Elsa's coronation ceremony, go for pink winterwear Anna, or you can go with casual Anna. Whatever you do, make sure you've got two red braids.
For Kristoff: Wear some gray, fur-lined clothes and a red belt, plus a little cap (and you get to wear UGGs!). Bonus points if you bring a stuffed Sven.