How to select Distinct / Group By child property in both LINQ method and syntax?

I have class

public class Parent
{
    [Key]
    public int ParentID { get; set;}
    public virtual ICollection<Child> Childs { get; set; }
}

public class Child
{
    [Key]
    public int ChildID { get; set; }
    public int Grade { get; set; }
    public int ParentID { get; set; }

    [ForeignKey("ParentID")]
    public virtual Parent Parent { get; set; }
}

I need something equivalent of

SELECT
    Child.Grade
FROM
    Parent
    INNER JOIN Child ON
        Parent.ParentID = Child.ParentID
WHERE
    Parent.ParentID = 1
GROUP BY
    Child.Grade

And

SELECT DISTINCT
    Child.Grade
FROM
    Parent
    INNER JOIN Child ON
        Parent.ParentID = Child.ParentID
WHERE
    Parent.ParentID = 1

Any help will be appreciated. Thanks