using System;

using System.Collections.Generic;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var p = new Parent() {Title="title1"};

        p.ChildItems = new List<ChildItem>();

        p.ChildItems.Add(new ChildItem() {ChildItemDetail = new ChildItemDetail() { Field1="1", Field2="2"}});

        p.ChildItems.Add(new ChildItem() {ChildItemDetail = new ChildItemDetail() { Field1="1", Field2="2"}});

        string json = JsonConvert.SerializeObject(p, new JsonSerializerSettings

            {

                DateFormatString = "MM/dd/yyyy",

                Formatting = Formatting.Indented

            });

        Console.WriteLine(json);

    }

    public class Parent

    {

        [JsonProperty("title")]

        public string Title { get; set; }

        [JsonProperty("child_items")]

        public List<ChildItem> ChildItems {get; set;}

    }

    public class ChildItem

    {

        [JsonProperty("child_item")]

        public ChildItemDetail ChildItemDetail {get;set;}

    }

    public class ChildItemDetail

    {

        [JsonProperty("field1")]

        public string Field1 { get; set; }

        [JsonProperty("field2")]

        public string Field2 { get; set; }

    }

}