You can take a child’s discount off by following these steps:
Children > Accounts > Discounts > Apply Discount to ‘None’ > Save.
*Note, you do not need to untick any boxes or take the discount amount out*. Once you have saved to None, you can check at the top right to ensure that both discounts show as £0.00. There will no longer be any discounts on that child’s account for any new invoices processed.