How to insert a Parent along with it's children in EF Core

In my ASP.NET Core 1.1.1 MVC web app, the Intellisense of the code editor of VS2017 does not recognize .child at line p.child.Add(c1);. What is the correct way of adding a Parent along with it's children.

...
Parent p = new Parent { Name = "some name" , FY = SelectedYear, ... };

Child c1 = new Child { ItemName = "Abc"};
Child c2 = new Child { ItemName = "Rst"};

p.child.Add(c1);
p..child.Add(c2);

_context.Add(p);
_context.SaveChanges();