Don't worry, I can instruct you.
Registering can be applied, registering to one Amazon.com account, to up to 6 reader devices.
I presume you haven't reached that limit?
To Register your new Kindle HD8:
- swipe down from the top of the screen, and select SETTINGS
- then tap on MY ACCOUNT
- then choose REGISTER
- then, choose If you already have an Amazon account:
- and enter your Amazon account username/email, then enter your Amazon account's password.
Submit that, and you should be registered, and have access in My Library to all the books you have already bought! without extra charge, and your daughter will have no account to reproach you for spending too much.