Hi,
First of all , let make sure the difference between domain admins and enterprise admins in parent domain (root domain).
Domain Admins:
Members of this group have full control of the domain. By default, this group is a member of the Administrators group on all domain controllers, all domain workstations, and all domain member servers at the time they are joined to the domain. By default, the Administrator account is a member of this group. Because the group has full control in the domain, add users with caution.
Enterprise Admins (only appears in the forest root domain)
Members of this group have full control of all domains in the forest. By default, this group is a member of the Administrators group on all domain controllers in the forest. By default, the Administrator account is a member of this group. Because this group has full control of the forest, add users with caution.
So a admin in the Domain Admins: in the parent domain will not have the permission to deletes users in child domain.
But admins in the Enterprise Admins will have the full permission to controller the child domain, include delete users in child domain.
Best Regards,