using System;

using System.Collections.Generic;

using System.Text.Json;

public class Program

{

    public static void Main()

    {

        string json = "{ \"Childs\": [{\"Test\":\"Foo\"}] }";

        var obj = System.Text.Json.JsonSerializer.Deserialize<A>(json, new JsonSerializerOptions ());

        Console.WriteLine(obj.Childs.Count);

        var s = JsonSerializer.Serialize(obj);

        Console.WriteLine(s);

    }

    class A

    {

        public List<B> Childs { get; set; } = new List<B>();

    }

    class B

    {

        public string Test {get;set;}

    }

}