Example of it in use:
Node topNode = new Node()
            {
                Value = "Top",
                Children = new List<Node>()
                {
                    new Node()
                    {
                        Value = "A",
                        Children = new List<Node>()
                        {
                            new Node()
                            {
                                Value = "AA",
                            },
                            new Node()
                            {
                                Value = "AB",
                            },
                            new Node()
                            {
                                Value = "AC",
                            }
                        }
                    },
                    new Node()
                    {
                        Value = "B",
                        Children = new List<Node>()
                        {
                            new Node()
                            {
                                Value = "BA",
                            },
                            new Node()
                            {
                                Value = "BB",
                            },
                            new Node()
                            {
                                Value = "BC",
                            }
                        }
                    },
                    new Node()
                    {
                        Value = "C",
                        Children = new List<Node>()
                        {
                            new Node()
                            {
                                Value = "CA",
                            },
                            new Node()
                            {
                                Value = "CB",
                            },
                            new Node()
                            {
                                Value = "CC",
                            }
                        }
                    },
                },
            };


            IEnumerable<Node> found = topNode.SearchTree((n) => n.Value == "BA", (n) => n.Children);

            foreach (var founditem in found)
            {
                Console.WriteLine(founditem.Value);
            }
Output:
BA

The actual extension method:
        public static IEnumerable<T> SearchTree<T>(this T Parent, Predicate<T> Condition, Func<T, IEnumerable<T>> FindChild)
        {
            List<T> found = new List<T>();
            foreach (var item in FindChild(Parent))
            {
                //Check the current node.
                if (Condition(item))
                {
                    found.Add(item);
                }

                //Search all children of the current node.
                IEnumerable<T> foundOnChild = SearchTree(item, Condition, FindChild);
                foreach (var foundItem in foundOnChild)
                {
                    found.Add(foundItem);
                }

            }

            return found;
        }