This is not correct. Apple actually requires that you use a credit card rather than a debit card in order to set up a child account for Family Sharing (there are related threads on discussions.apple.com but I'm not able to add links here). I just attempted this. It's frustrating because our family tries to avoid using a credit card. As the linked thread points out, you can't (currently) just use the credit card for verification purposes, you must use it as your method of payment on an ongoing basis. Apple doesn't allow multiple cards to be added.
I also noticed that if you try to change your account in the OS X preference panel for iCloud, this warning is displayed:
So Apple requires you to pay with a credit card in order to share purchases, but if you switch to one you won't be able to share purchases for 3 months.

Well done, Apple.
