Q: Apple says my credit card is a debit card

Is it a debit card or a credit card - there are visa debit cards as well as visa credit cards.

Due to child online privacy protection laws debit cards are not a valid payment method when trying to create a child account : Family Sharing and Apple ID for your child - Apple Support

Before you begin, make sure that you're using a credit card or UnionPay as your iTunes Store and App Store payment method. To comply with child online privacy protection laws, you will use the CVV, verification code sent via SMS, or security code from a valid payment method as part of providing your parental consent. If the card on file is a debit card or another payment method, you’ll be asked to provide a valid payment method before you can continue. After you create the child's Apple ID, you can change your payment method.

What does 'It is an ordinary VISA card' mean ? There are visa debit cards as well as visa credit cards.

If it's being recognised as a debit card then you won't be able to use it as a payment method for creating a child account. Do you have a card that is recognised as being a credit card ?