If I'm getting your problem right, then you can and should save the child as a simple Sometype with a parentId, just as you would without using the entity framework. The automagic of the EF shouldn't cause a problem for you here, and you would not be breaking any best practices.
Essentially: If the parent entity is already loaded into context, it's usually best to add the new entity to the collection of navigation properties (Children in your case, but any foreign key). If the parent is not loaded and you know it's id, populate the foreign key field and save it just as you always did.