How do I find my daughters I phone?

If you mean cellular plan, it has nothing to do with finding the iPhone.

If you enabled Find My iPhone in iCloud settings of the lost phone, you can go to the Find My iPhone App in another iOS device, Mac or the iCloud.com website, and log in with the Apple ID used to setup iCloud on the lost iPhone.

If the iPhone is turned on, has an internet connection (either wifi or cellular), and Find My iPhone was turned on in settings as above, then it should be traceable.

More details here:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

If you have Find my Iphone turned on (on your daughter device) you can locate it.

If you're asking about a differnt method, you can use this support guide:

About Find My Friends - Apple Support

Regards

Giulio

You don’t need the device password, you need to know the password to her iCloud account. Otherwise, you cannot log into find my iPhone as her and track her devices.

It is impossible to track a device linked to a specific iCloud account without being able to log in to that account.

As said, you need the Apple ID password. Ostensibly, this should be a password you set up if she's a minor.