Implementing a tree in c# managing parent-child

public class Folder
{
    //Various Props like Name etc.
    public IList<Folder> Children{get;}
    public Folder Parent {get;}
}

List<Folder> children;

public ReadOnlyCollection<Folder> Children
{
    get { return this.children.AsReadOnly(); }
}

IList<Folder> children;

public ReadOnlyCollection<Folder> Children
{
    get
    {
        return new List<Folder>(this.children).AsReadOnly();
    }
}

public class ChildCollection
{
   // _Children is maintained by the Folder class, hence the internal access specifier
   internal Dictionary<KeyType, Folder> _Children = new Dictionary<KeyType, Folder>;

   public this[KeyType key]
   {
      get
      {
          return _Children[key];
      }
   }

   public IEnumerable<KeyType> Keys
   {
      get
      {
         return _Children.Keys;
      }
   }
}

public IEnumerable<Folder> Children
    {
        get { return this.children.GetEnumerator(); }
    }