A Receipt is not a notification. The Family Organizer will not get an email or system notification telling them an in-app purchase was made.
A receipt is issued only to the account that makes the purchase, not the Family Organizer.
The Family Organizer does not get receipts for things purchased by other members at all. The receipt for those purchases appears only on the member's account that made the purchase even if it was charged to the Family Organizer.
They would see the charge on their payment method's statement if any amount is charged to their payment method. Since you are using your own gift card balance, they will not see a charge on their payment method nor a receipt on their account.
They can still open the Family Member's purchases list and see the receipt there after the fact though.
If you are so worried about them finding out, then maybe you should re-consider making the purchase.