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.