using System;
using System.Linq;
using Microsoft.Commerce.Common.MessageBuilders;
using Microsoft.Commerce.Contracts;
using Microsoft.Commerce.Contracts.Messages;
using Microsoft.CommerceServer.Catalog;
using Microsoft.CommerceServer.Runtime;

namespace ASPNet3346
{
    public partial class GetChildProducts : System.Web.UI.Page
    {
        protected void btnCS2007_Click(object sender, EventArgs e)
        {
            ProductCatalog productCatalog = (ProductCatalog)CommerceContext.Current.CatalogSystem.GetCatalog("Adventure Works Catalog", "en-US");
            // Get the root category.
            Category departmentCategory = productCatalog.GetCategory("Boots");
            ProductCollection childProducts = departmentCategory.ChildProducts;
            foreach (Product product in childProducts)
            {
                DisplayHelper.DisplayValue("DisplayName", product.DisplayName, Response);
            }
        }

        protected void btnCS2009_Click(object sender, EventArgs e)
        {

            var catalogQuery = new CommerceQuery("Category");
            catalogQuery.SearchCriteria.Model.Id = "Boots";
            catalogQuery.SearchCriteria.Model.Properties["CatalogId"] = "Adventure Works Catalog";

            //Get Child Categories
            // Add Related Query Operation for child products
            {
                CommerceQueryRelatedItem queryChildProducts =
                    new CommerceQueryRelatedItem("ChildProducts", "Product");
                catalogQuery.RelatedOperations.Add(queryChildProducts);
            }
            CommerceResponse response = CommerceFoundationServiceAgent.Execute(catalogQuery);

            // Handle Responses
            CommerceQueryOperationResponse queryResponse = response.OperationResponses.Single() as CommerceQueryOperationResponse;
            CommerceEntity productCatalog = queryResponse.CommerceEntities.SingleOrDefault();
            var childCategories = productCatalog.GetPropertyValue("ChildProducts") as CommerceRelationshipList;
            if (childCategories != null)
                foreach (var commerceRelationship in childCategories)
                {
                    CommerceEntity childcategory = commerceRelationship.Target;
                    DisplayHelper.DisplayValue("DisplayName", childcategory.Properties["DisplayName"], Response);
                }
        }
    }
}

Attached Source Code : ASPNet3346