Good day zotje,
My understanding of your question is that you have a Family Sharing group of which you are the organizer or approver, and that children in the group are using Ask to Buy. You want to know whether they will have to ask when using a balance from a gift card to make a purchase. Is that correct?
If a Family Sharing member has Ask to Buy enabled for them, all purchases must be approved, regardless of account balance status. See this article for reference - Request and make purchases with Ask to Buy.
Specifically -
When a family member initiates a new purchase or free download, a request is sent to the organizer if Ask to Buy is on.
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Safe computing.