Using iTunes gift cards with family sharing

I have a family share plan for my kids. They received iTunes gift cards. They would like to buy an app that costs $. When they make their request it asks to verify my credit card and I do not see a way to pay for their app with my gift card balance.

The iTunes store on my Mac doesnt even show the option to purchase apps (just movies, tv, music, etc).

How do I use an iTunes gift card to pay for my kids purchases (they have their own apple ID under MY family account).

Thanks!