#models.py
class Material(models.Model):
    name = models.CharField()

class Parent(models.Model):
    ...

class Child1(Parent):
    material = models.ForeignKey(Material,null=True, blank=True, on_delete=models.SET_NULL)

class Child2(Parent):
    material = models.ForeignKey(Material,null=True, blank=True, on_delete=models.SET_NULL)

class Child3(Parent):
    #нет поля material


#views.py
def foo(search_material):
    parents = Parent.objects.all()
    parents_list = []
    for parent in parents:
        parent_obj = parent.obj
        if hasattr(parent_obj,'material'):
            if parent_obj.material.name.upper().find(search_material.upper())==0:
                parents_list.append(parent_obj)
    return parents_list