Hello,
this is not possible using only permission customization. First of all, CreateChildren / LinkChildren are permissions on the parent and do not restrict the potential to create children of specific types. Furthermore, CreateChildren / LinkChildren are defined on the sys:base type and can (should) not be redefined to another type without causing major issues with other parts of Alfresco.
You would need to work with a combination of behaviour / policy implementation, user interface configuration and custom permissions to restrict the ability of users to create new children for specific types…
Regards
Axel