using System;

using System.Collections.Generic;   

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var p = new List<Parent> {

                new Parent {

                 ParentName = "Parent 1",

                 Childs = new List<Child> {

                    new Child {

                        ChildName = "Child1-1"

                    },

                    new Child {

                     ChildName = "Child1-2"

                    }

                 }

                },

                new Parent {

                 ParentName = "Parent 2",

                 Childs = new List<Child> {

                    new Child {

                        ChildName = "Child2-1"

                    },

                    new Child {

                     ChildName = "Child2-2"

                    }

                 }

                }

            };

​

            var obj = p.SelectMany(t => t.Childs.Select(m => new { m.ChildName, t.ParentName })).ToList();

​

            foreach (var item in obj) {

                Console.WriteLine($"ParentName: {item.ParentName}, ChildName: {item.ChildName}");

            }

    }

}

public class Parent

    {

        public string ParentName { get; set; }

        public List<Child> Childs { get; set; }

    }

​

    public class Child

    {

        public string ChildName { get; set; }

    }