to credit xbox account you need to add funds to your childs account

• Go to your family group and sign in with your Microsoft account.
• Find your child’s name. Select the name and then select Add money. If that option is not available, select Options > Spending > Microsoft account balance > Add money.
• There are several preset amounts to choose from. Your Microsoft account balance and gift cards are non-transferable to another person or Microsoft account. For more info, see Microsoft gift cards terms and conditions.