Should classes, that modify the state of the parent class, but not itself, be unit tested separately? And by separately, I mean putting the test in the corresponding unit test class, that tests that specific class.

I'm developing a library based on chained methods, that return a new instance of a new type in most cases, where a chained method is called. The returned instances only modify the root parent state, but not itself.

Overly simplified example, to get the point across:

    public class BoxedRabbits
    {
        private readonly Box _box;
        public BoxedRabbits(Box box)
        {
            _box = box;
        }

        public void SetCount(int count)
        {
            _box.Items += count;
        }
    }

    public class Box
    {
        public int Items { get; set; }
        public BoxedRabbits AddRabbits()
        {
            return new BoxedRabbits(this);
        }
    }

var box = new Box();
box.AddRabbits().SetCount(14);

Say, if I write a unit test under the Box class unit tests:

box.AddRabbits().SetCount(14)

I could effectively say, that I've already tested the BoxedRabbits class as well. Is this the wrong way of approaching this, even though it's far simpler to first write a test for the above call, then to first write a unit test for the BoxedRabbits separately?