The following syntax should work:

       children: [

             { label: "Leaf One" },

             { label: "Leaf Two" },

             { label: "Subtree", children: [

                  {  label: "Subtree - Leaf One" },

                  {  label: "Subtree - Leaf Two" },

                  {  label: "Subtree - Leaf Three" },

                  {  label: "Subtree - Leaf Four" }

             ]}

      ]

Developers should not have to specify an explicit type when creating a subtree unless they are doing something unusual.