Hi Eric,
if you do not know the password of her keychain then there is no way to change the password. This is what makes the keychain so secure: no password - no access.
By default, both your login and your keychain password are the same. Once you log into your account the keychain is opened and all applications that need a particular password to perform their duty can automatically retrieve that password from the keychain without any user interaction.
However, once your keychain has a different password it will not open on login. The applications will still try to access the stored informations but cannot do so, without giving them the keychain password. That is why these windows pop up, asking you for the keychain's password.
So, either you come up with the correct password and then change it to one that matches your daughter's login password or you will have to delete the keychain: In Keycain Access, choose Keychain List from the Edit menu and then delete the "login" keychain. Deleting the keychain removes the keychain from the Keychains list but does not remove the keychain file from your disk. If you remember your password later, you can recover the items stored in the keychain by choosing Edit > Keychain List and adding this file.
-Petra