how do i add a second iphone to my Find My Phone account or

The Find My Phone App works by logging in to a specific Apple ID. It will track devices that are using that Apple ID for iCloud.

You can log in with your daughter's Apple ID to the Find My iPhone App on your iPhone or iPad and it can track her device. Note, the Find My iPhone App is different to the Find My iPhone Setting in Settings->Accounts(top of Settings)->iCloud->Find My iPhone.

The other option is to use Family Sharing and have her share her location:

Read here:

Share your location with your family - Apple Support

If she has a different ID than you, you would have to log in using her Apple ID/password.

Family Sharing

Family Sharing -What types of content can I share?

Family Sharing - If you don‘t see your family‘s shared content

Family Sharing - purchases and payments

Family Sharing – share iCloud Storage

https://support.apple.com/en-us/HT205362