Can't share my card with family members, have to buy everything twice
You shouldn't need to pay for other family members to download the app, provided you all are running the latest iOS software (I'm not sure if it requires iOS 9), iCloud Sharing should allow purchases to be shared between six family members. See http://www.apple.com/icloud/family-sharing/ for information and https://support.apple.com/en-us/HT201088 for help setting it up.

You can go to the receipt Apple will send you for the $20 Keynote purchase, and there will be a link in there to report a problem, and you can indicate you did not mean to purchase. It will hopefully result in a refund.