as mentioned by ypercube your use case doesn’t require subquery.
but anyway since many people land into this page to learn how to do sub-query here is how its done.
employee_query = Employee.objects.filter(company='Private').only('id').all()
Person.objects.value('name', 'age').filter(id__in=employee_query)
Source:
http://mattrobenolt.com/the-django-orm-and-subqueries/