Q: how can I use separate devices on my account

Use Family Sharing

http://www.apple.com/ios/ios8/family-sharing/

Family purchases and payments - Apple Support

Also, as necessary:

Family Sharing and Apple ID for your child - Apple Support

http://www.macstories.net/tutorials/ios-8-family-sharing-explained-share-purchas es-between-family-members-much-more/

She has to have her own ID and she just redeems the card into her ID/account.

After you set up your family, any time a family member initiates a new purchase it will be billed directly to your account, unless that family member has gift or store credit. First, their store credit will be used to pay the partial or total bill. The remainder will bill to the family organizer's card. As the family organizer, any receipts generated by the transaction will be sent to you. Learn more about how iTunes Store purchases are billed.