Note: Nevertheless, a credit card required! (UPDATE)

I have added to the account of my child a credit card.
Nevertheless, no purchase was possible.
After consultation with the technical support from Microsoft I received the following Aufkunft:

It must nevertheless necessarily be deposited in the payment information of the Microsoft account a credit card!
Why then a prepaid card is good, was able to get the Microsoft employee also not explain.

Now the 25 euros will stagnate until doomsday in the account.

UPDATE:

Problem was that the purchase was made through the Web browser, and more than a mobile phone is associated with the account.
One of them still using Windows 7.8. For this target platform with a Pay credit card is not possible. (Even via the web front end of the store)

Now the desired app was available for all platforms (7.8, 8.0, 8.1)
As a mobile phone with 7.8 still is associated with the account, and the version for this "old" device is automatically acquired when buying. This probably prevented the payment by credit