How to build a nested tree with ASP.NET Web API

I'm using ASP.NET Web API 2 for the backend and AngularJS for the frontend. I want to display a treeview in AngularJS.

So I need a nested Model in my ASP Web API. The model currently looks like this:

public class Register
{
    public int ID { get; set; }
    public string Name { get; set; }
    public List<Register> Children { get; set; }
}

and the controller like this:

    // GET: api/Registers
    public IQueryable<Register> GetRegister()
    {
        return db.Register;
    }

In my Database I added following examples:

The JSON Output shows:

[{"ID":4,"Name":"RootFolder","Children":[{"ID":5,"Name":"Sub1","Children":null}]},{"ID":5,"Name":"Sub1","Children":null}]

The first entry looks fine, but I don't want to output the children itself. It it possible to only output the RootFolder with its Children and not the children itself? Like this:

{"ID":4,"Name":"RootFolder","Children":[{"ID":5,"Name":"Sub1","Children":null}]}

The second problem is, when I want enter a specific Folder ID (For example ID 4(RootFolder)). I only get this as a response:

Why there are no child elements? The controller is this one:

     // GET: api/Registers/5
    [ResponseType(typeof(Register))]
    public IHttpActionResult GetRegister(int id)
    {
        Register register = db.Register.Find(id);
        if (register == null)
        {
            return NotFound();
        }

        return Ok(register);
    }

Summary:

• How can I filter the Output to only show the RootFolders with each children but not showing the children objects itself?
• Why there are no children when I specify a specific ID?