Hello.
Regrettably, this functionality is not provided in current version of Kentico
CMS. You could try customization via CustomDataHandler class (
http://devnet.kentico.com/docs/devguide/data_handler.htm) and check number of actual child documents of given parent in OnBeforeInsert method. However, this is up to you to implement such functionality via our API and customization possibilities.
Best Regards,
Radek Macalik