public bool IsRoot => Parent == null;

public int Count => Children.Count;

public void AddChild(Node node)
{
    node = node ?? throw new ArgumentNullException(nameof(node));
    if (IsAncestorOrSelf(node)) // <- you should implement such method
        throw new ArgumentException("The node can not be an ancestor or self");
    if (IsDescendant(node)) // <- you should implement such method
        throw new ArgumentException("The node can not be a descendant");
    node.Parent = this;
    Children.Add(node);
}

public IEnumerable<Node> GetChildren()
{
    return Children.ToArray();
}

public IEnumerable<Node> GetDescendants()
{
    foreach (var child in Children)
    {
         yield return child;
         foreach (var descendant in child.GetDescendants())
         {
              yield return descendant;
         }
    }
}

public Node DeepCopy()
{
    var other = (Node)MemberwiseClone();

    other.Children = new List<Node>(collection: Children);
    other.Parent = Parent?.DeepCopy();
    other.Value = new Node(value: Value);

    return other;
}

public void RemoveChild(Node node)
{
    if (node != this && Children.Contains(node))
    {
        Children.Remove(node);
    }
}

public class Node<T>
{
    public T Value { get; }
    ...
}