You could turn off 'ask to buy' for her account (Request and make purchases with Ask to Buy - Apple Support), and instead not let her know her account's password (if she already knows it you could change it via http://appleid.apple.com ; What to do after you change your Apple ID email address or password - Apple Support) - you would then have to enter it to buy/download anything new.
If you do that there are a couple of settings on her iPod that you might want to check/change, all are in Settings > General > Restrictions (you need to have Restrictions enabled to change/set them ; the Restrictions passcode doesn't, and probably shouldn't be as I assume that your daughter will know it, the same as the passcode to unlock the iPod)
- Password Settings . The default might be '15 minutes' which means that the account's password doesn't have to be re-entered for 15 minutes after entering, potentially allowing further purchases without it.
- Installing Apps - that will hide the App Store app so that the store can't be browsed and therefore items downloaded. When she potentially wants something you could turn that back 'on' to unhide the app and supervise her browsing/downloading.
- iTunes Store - similarly hides the iTunes Store app
- In-App Purchases - prevent in-app purchases being done in apps
- there is also an 'allowed contend' section, if you aren't (or potentially even if you are) going to hide the store apps you may want to block explicit music, and age-rated films/TV programmes - they will still show in the store, but she won't be able to download them nor play their previews in the store.
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