This example returns the child Web site with the specified leaf URL.

PnPSitesCore.Samles.getweb.cs

//Author: Shantha Kumar T
//Assembly Reference Used: OfficeDevPnP.Core, Version=2.5.1606.1, Culture=neutral, PublicKeyToken=3751622786b357c2
//Supports: SharePoint Online, SharePoint 2013+

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Microsoft.SharePoint.Client;
using OfficeDevPnP.Core;

// Assembly Reference Used: OfficeDevPnP.Core, Version=2.5.1606.1, Culture=neutral, PublicKeyToken=3751622786b357c2
namespace PnPSitesCore.Samples
{
    class Program
    {
        static void Main(string[] args)
        {
            string siteUrl = "https://sharepointonline.sharepoint.com";           

            AuthenticationManager authManager = new AuthenticationManager();
            //Interactive Login to SharePoint site - Opens a Online signin page to authenticate the user
            var context = authManager.GetWebLoginClientContext(siteUrl);
	          
	          //Returns the server-relative url of the instantiated web object
            Web oweb = context.Web.GetWeb("subsite");
            Console.WriteLine(oweb.ServerRelativeUrl);

            Console.WriteLine("Press any key to exit.");
            Console.ReadKey();
            
        } 
    }
}