Parent class access modifier

Question

I have an abstract ParentClass which contains a private PrivateThing class which I don't want to expose to derived classes, to which I instead provide ThingContainer.

However, there's no access modifier to make ThingContainer.Things visible to ParentClass;

protected/internal:

• results in a compiler error, since PrivateThing is private.
• would expose ThingContainer.Things to derived classes as well, which is undesireable.

Is there a way to make ThingContainer.Things visible to the ParentClass without exposing PrivateThing?

public abstract class ParentClass
{
    private sealed class PrivateThing { }

    protected sealed class ThingContainer
    {
        private List<PrivateThing> _things = new();

        // Should be visible to ParentClass
        private IReadOnlyList<PrivateThing> Things => _things;

        public void AddThing()
        {
            _things.Add(new PrivateThing());
        }
    }


    private readonly List<ThingContainer> _thingContainers = new();

    protected void Setup(Action<ThingContainer> setupThings)
    {
        var container = new ThingContainer();
        setupThings(container);
        _thingContainers.Add(container);
    }

    public void ProcessThings()
    {
        foreach (var thing in _thingContainers.SelectMany(static b => b.Things /* Can't access */))
            Console.WriteLine(thing.ToString());
    }
}

public sealed class DerivedClass : ParentClass
{
    public DerivedClass()
    {
        Setup(static things =>
        {
            things.AddThing();
        });
    }
}

Edit: Answered by Ralf in the comments, thanks!