The Wrong Night, by Katrine Robinson

A lovely book, ideal for when they get to reading proper stories and have moved away from the very short stuff. Easy to empathise with the main character, Percy, and his problems at school etc - and the grumpy Santa is fantastic fun as the story develops. We loved the reindeer with attention deficit disorder too. Pity it's only available as an Amazon ebook so you need to read it on a PC or Kindle or something, but worth it.