allow free purchases for children

How can I allow my children to purchase free apps as part of family sharing but require approval for all applications that are not free? My children buy a lot of free apps, and have iTunes credit but since they are part of my family they now also have access to my credit card... I would like them to be able to buy anything free without my approval (since I don't want to approve free apps all day long having many children) but have to ask for approval for all charged apps.

thanks.

p