App.cs

var currentItem = context.GetCurrentItem<Parent>();

var newChild = new Child();
//we use the Name property as the item name when creating an item
newChild.Name = "MyNewChild";

//you can map other properties here
newChild.PageTitle ="My New Child";

using(new SecurityDisabler()){
  context.Create(parent, child); 
}

Models.cs

[SitecoreClass]
public class Parent
{
   [SitecoreId]
   public virtual Guid Id{get;set;}
}

[SitecoreClass]
public class Child
{
  [SitecoreId]
  public virtual Guid Id{get;set;}
  
  [SitecoreInfo(SitecoreInfoType.Name)]
  public virtual string Name{get;set;}
  
  [SitecoreField]
  public virtual string PageTitle{get;set;}
}