Database structure with Entity Framework Core - Discrimination - asp.net-core

Question

I have the following problem:
I'd like to be able sell a few kinds of products in my store.
I have created following infrastructure:
public class Product
{
public int Id {get; set;}
public string Name {get; set;}
...
public int? ParentId {get; set;}
public Product Parent {get; set;}
public ICollection<Product> Children {get; set;}
}
public class AudioFile : Product
{
public string SomeValue {get; set;}
}
public class DocummentFile : Product
{
public string SomeString {get; set;}
}
In fluent mapping I am using discriminator.
So, to Select the amount of documment files without a parent, I am doing something like this:
DbContext.DocummentFiles.Count(x=> !x.ParentId.HasValue)
It works with Audio files too.
I'd like to select sample dictionary of all children for some audio file with SomeValue:
DbContext.AudioFiles
.Where(x => x.ParentId == 222)
.Select(x => x.Children
.OfType<AudioFile>()
.ToDictionary(t => t.Name, t => t.SomeValue));
Unfortunately, it does not work. I have high CPU usage by the long time, but I have not any result.