Change iMac iTunes User Account synching with my ATV

If all of the files are in the same iTunes library, you should be able to log into your iTunes account, then authorize the files from her user account with her user name and password when prompted to do so. If the files are in separate iTunes accounts, you could use Home Sharing to add her files to your library, but both libraries would need to be online at the same time.

settings/computers from the tv menu and select your daughters library, click and follow instructions to remove the library.

The entry in settings/computers will now change from your daughters library to connect to itunes, click on this and follow the instructions to pair the tv with your library.

Follow capahos instructions for moving and authorising any content in your daughters itunes library to your own, or you can simply drag and drop content from one account to the other accounts drop box and manually add it to your account. Click to play newly added content, the first time you do this you will need to enter your daughters itunes account details.

If you have any content which has been purchased on the tv under your daughters account and has not transferred back to her library, force this to transfer before unpairing her library.

settings/computers from the @tv menu and select your daughters library, click and follow instructions to remove the library.

If I'm reading the OP's post correctly, they have two iTunes user accounts in iTunes on the same computer, which means all of the files are in the same library. If that's the case, then it's merely an authorization issue.

You may be correct, I'm assuming each itunes account is on a separate user account and that each account uses its own user/music/itunes/itunes music folder.

Fortunately the OP has both scenarios covered with the answers provided.

You may be correct, I'm assuming each itunes account is on a separate user account and that each account uses its own user/music/itunes/itunes music folder.

The location setting for the iTunes library in iTunes preferences is for the computer user account on the computer, not the iTunes store account. If they have two iTunes store users in the same iTunes installation, all of the files will be in the same library.

I'm not disagreeing with you, and you are of course correct if indeed the two itunes store accounts are used from the same user account. I got the feeling the OP had separate user accounts (as in 2 OSX user accounts)