Apple Family Billing Issues

My husband is our account organizer and purchase sharing is turned off. My credit card is connected to my son's apple id. When he makes a purchase he receives a receipt and it charges my card. Yet, my husband also receives a receipt and it ALSO charges his debit card. It took a while for us to catch it since it was on two separate cards. It seems this has been happening for over a year. I reached out to Apple and they only refunded anything double charged in the last 90 days. I asked how to prevent it in the future and was told they didn't know and the person I spoke to was the highest level so there was nothing they could do. She said on their end it looked like two separate purchases so they couldn't help. How do we get this to stop? We want to keep the family set up because it helps us monitor our children's phones but this is just ridiculous.