Argos has a couple of really cute Disney Frozen bags for kids at the moment, at decent prices and with free delivery. Do note that they're only available for home delivery so you'll need to buy one online - you can't pick them up off the shelf in store.
There's a third off the price of each, so the holdall is down from £14.99 to £9.98, and the Backpack is down from £12.99 to £8.65.
I think either of these would make a lovely Christmas present for a little Disney Frozen fan - you could always fill it with Disney Frozen pyjamas or other movie-related treats and surprises. The holdall is especially cute - I know my kids used to love having their own bags that were like grown up bags as opposed to school backpacks, and it would be ideal for sleepovers or holidays. The backpack is more suited to a first school bag, I think, but with a third off the price and free delivery I don't imagine they'll stay in stock for long - so be quick if you're hoping to nab one!