Try this,
using umbraco.cms.businesslogic.web;
DocumentType dt = DocumentType.GetByAlias("alias");
// The umbraco user that should create the document,
// 0 is the umbraco system user, and always exists
umbraco.BusinessLogic.User u = new umbraco.BusinessLogic.User(0);
//Replace 1055 with id of parent node
Document doc = Document.MakeNew("new child node name", dt, u, 1055);
//after creating the document, prepare it for publishing
doc.Publish(u);
//Tell umbraco to publish the document
umbraco.library.UpdateDocumentCache(doc.Id);
OR
using Umbraco.Core;
using Umbraco.Core.Models;
using Umbraco.Core.Services;
// Get the Umbraco Content Service
var contentService = Services.ContentService;
var product = contentService.CreateContent(
"my new presentation", // the name of the product
1055, // the parent id should be the id of the group node
"Presentation", // the alias of the product Document Type
0);
// We need to update properties (product id, original name and the price)
product.SetValue("title", "My new presentation");
// finally we need to save and publish it (which also saves the product!)
// - that's done via the Content Service
contentService.SaveAndPublish(product);
Hope this helps