теперь можно так:

        public class Parent
        {
            [MapField("ParentID"), /*[a]*/PrimaryKey/*[/a]*/]
            public int ID;

            /*[a]*/[Relation(typeof(Child))]/*[/a]*/
            public List<Child> Children = new List<Child>();
        }

        [MapField("ParentID", "Parent.ID")]
        public class Child
        {
            [MapField("ChildID"), /*[a]*/PrimaryKey/*[/a]*/]
            public int ID;

            /*[a]*/[Relation]/*[/a]*/
            public Parent Parent = new Parent();

            /*[a]*/[Relation(typeof(Grandchild))]/*[/a]*/
            public List<Grandchild> Grandchildren = new List<Grandchild>();
        }

        [MapField("ChildID", "Child.ID")]
        public class Grandchild
        {
            [MapField("GrandchildID"), /*[a]*/PrimaryKey/*[/a]*/]
            public int ID;

            /*[a]*/[Relation]/*[/a]*/
            public Child Child = new Child();
        }
        [Test]
        public void Test2()
        {
            List<Parent> parents = new List<Parent>();
            /*[a]*/MapResultSet/*[/a]*/[] sets = new MapResultSet[3];

            sets[0] = new MapResultSet(typeof(Parent), parents);
            sets[1] = new MapResultSet(typeof(Child));
            sets[2] = new MapResultSet(typeof(Grandchild));

            using (DbManager db = new DbManager())
            {
                db
                    .SetCommand(TestQuery)
                    ./*[a]*/ExecuteResultSet/*[/a]*/(sets);
            }

            Assert.IsNotEmpty(parents);
        //...
        }

по умолчанию индексы берутся по PrimaryKey, можно задать непосредственно аттрибутом.