Why can’t I add it to my granddaughters account.
Because it is not possible to use a credit card for 2 accounts OR you can set up Family Sharing.
You can buy an Apple Gift Card or App Store & iTunes gift card for her, so she can use her Apple ID balance as payment method.
See -> How to redeem your Apple Gift Card or App Store & iTunes gift card
How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)
What you can buy with your Apple Gift Card or Apple ID balance
What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)
See -> Check your gift card balance
Check your gift card balance - Apple Support
OR on your iPhone -> Tap App Store -> tap your profile (top right corner of the screen).
Or see -> Check your Apple ID balance
Check your Apple ID balance – Apple Support (UK)