есть идея сделать так:

using (DbManager db = new DbManager())
{
    List<Parent> parents = db
        .AddCommand<Parent>    ("SELECT * FROM Parent")
        .AddCommand<Child>     ("SELECT * FROM Child")
        .AddCommand<GrandChild>("SELECT * FROM GrandChild")
        .ExecuteRelations<Parent>();
}

в перспективе, думаю, можно будет делать так:

public abstract calss ParentAccessor : DataAccessor<Parent, ParentAccessor>
{
    [Command(typeof(Parent),     "SELECT * FROM Parent")]
    [Command(typeof(Child),      "SELECT * FROM Child")]
    [Command(typeof(GrandChild), "SELECT * FROM GrandChild")]
    public abstract List<Parent> SelectAll();
}