using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        string json = @"{

    ""Name"": ""Eli"",

    ""Age"": 4,

    ""Children"": {

        ""Moshe"": {

            ""Age"": 6,

            ""Characteristics"": ""Nice;Good;""

        },

        ""Yossi"": {

            ""Age"": 3,

            ""Characteristics"": ""Hero;Brave""

        }

    }

}";

        string result = Regex.Replace(json, @"(\""Children\""\:(.|\n)*?)(\{(.|\n)*\{(.|\n)*?\}(.|\n)*?\})", (m) =>

        {

            return (m.Groups[1] + "\"" + Regex.Replace(m.Groups[3].Value.Replace("\n", " "), @"\s+", " ") + "\"");

        }

​

        );

        Console.WriteLine(result);

    }

}