Q: Apple IDs for kids with a debit card

As far as I know Apple haven't said why debit cards aren't accepted, all we know is what it says on the page that you posted from i.e. that you have to have a credit card  :

Before you begin, make sure that you're using a credit card as your iTunes Store and App Store payment method.

Edit :

Though Family purchases and payments says :

When you set up Family Sharing, you'll be asked to confirm that you agree to pay for purchases initiated by the family members you invite and that you have a valid payment method on file. Valid payment methods for setting up Family Sharing include credit cards and debit cards.

So it appears to contradict - but debit cards don't appear to be still be an accepted payment method in all countries.