Parent class with the same responsibility as the child class

You can simplify for one class only:

public class Item
{
    [Key]
    public int ItemId { get; set; }
    public int? ItemPaiId { get; set; }

    public string Nome {get;set;}
    public decimal Valor {get;set;}

    public virtual Item ItemPai { get; set; }
    public virtual ICollection<Item> SubItens {get;set;}
}

I think it solves the whole problem.

I do not know what the purpose of class is, but starting from your code, I think this is enough:

public class Item
{
   public string Nome {get;set;}
   public decimal Valor {get;set;}

   public ICollection<Item> SubItens {get;set;}
}

There is no need for a SubItem class because a SubItem is no more than a item .
Conclusion you've already taken when you've acknowledged that Item and SubItem have the same responsibility     

Since the Item class is structured, even applying the @ramaral solution, it will be very difficult for the client to use the item tree. The client will always have to check if an item is a tree node ( if(item.SubItems != null) ) or if the item is a leaf of the tree.

For example, to print the entire tree, the client would have to define a recursive method, and at each iteration determine the item type.

public void PrintNode(Item item)
{
    if(item.SubItems == null)               //é folha?
        Console.WriteLine(item.Name);
    else                                    //é nó?
        foreach(var subItem in item.SubItems)
            PrintNode(subItem);
}

Item root = //criar a arvore
PrintNode(root);

Ideally, you should restructure the Item class to expose only manipulation methods, and apply the Composite < a> in order to treat all items, be it knots or sheets, evenly.

public interface IItem
{
    void Do(Action<Item> action);
}

public class ItemComposite : IItem
{
    private readonly IEnumerable<IItem> _subItems;

    public ItemComposite(IEnumerable<IItem> subItems)
    {
        _subItems = subItems;
    }

    public void Do(Action<Item> action)
    {
        foreach(var subItem in _subItems)
            subItem.Do(action);
    }
}

public class Item : IItem
{
    public string Nome {get; private set;}
    public decimal Valor {get; private set;}

    public Item(string nome, decimal valor)
    {
        Nome = nome;
        Valor = valor;
    }

    public void Do(Action<Item> action)
    {
        action(this);
    }
}

Client code to print the tree:

IItem root = //construir árvore 
root.Do(item => Console.WriteLine(item.Nome));

Another possible implementation with support for IEnumerable<T>

public interface IItem : IEnumerable<Item>
{
}

public class ItemComposite : IItem
{
    private readonly IEnumerable<IItem> _subItems;

    public ItemComposite(IEnumerable<IItem> subItems)
    {
        _subItems = subItems;
    }

    public IEnumerator<Item> GetEnumerator()
    {
        return _subItems.SelectMany(subItem => subItem).GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

public class Item : IItem
{
    public string Nome { get; private set; }
    public decimal Valor { get; private set; }

    public Item(string nome, decimal valor)
    {
        Nome = nome;
        Valor = valor;
    }

    public IEnumerator<Item> GetEnumerator()
    {
        yield return this;
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

Customer Code

IItem root = //...

foreach(Item item in root)
    Console.WriteLine(item.Nome);

You can put a small logic in get of Valor of Item .

public class Item
{
    private decimal valor;
    public string Nome {get;set;}
    public decimal Valor 
    {
        get
        {
            if (this.SubItens != null && SubItens.Any())
                return this.SubItens.Select(subItem => subItem.Valor).Sum();
            return this.valor;
        }
        set
        {
            this.valor = value;
        }
    }

    public ICollection<SubItem> SubItens {get;set;}
}

public class SubItem 
{
    public string Nome {get;set;}
    public decimal Valor {get;set;}
}