We have two children, now 18 year old twins, who have been on our Family Sharing function. We would like to now have them pay for their own purchases.
How do we do that?
You can't purely, Family Sharing requires the organizer to guarantee payment for everybody. However, if any member has sufficient balance from, say, a gift card, that balance will be used first. Tell them to always have enough balance or else there will be words...
How iTunes Store purchases are billed - https://support.apple.com/HT201359 --> About Family Sharing purchases
If they are still on AskToBuy I think you can just leave it turned on, but that doesn't force them to pay if you approve a purchase.
Family sharing: How to set up a payment method for an 18 year old child but keep them on family sharing
