I have a Kindle myself, have just bought one for my daughter and I'm not sure how to set up the second. Do I register it to my current account or reregister to a different account? Would registering on my account mix up the book downloads on the devices? She's 10 (and very much a reader!) so I'd be buying any books for her at this stage.

You register to your account, there should be the option to add a kindle. It won't mix your books but you will be able to download already purchased books to the new device.

Ok thanks!