Will you have access to wifi while away? If so then
www.oxfordowl.co.uk is amazing. You'll need a flash app like Puffin to read the books though, don't think you can download them.
You should also have a look at the reading section of
www.smartappsforkids.com. They have a fantastic collection. You can sign up for their Free Apps newsletter and they'll send you links to great apps that are free for 24hrs. There's often good book apps going free for limited time...