When attempting to introduce a plane into the scene, I noticed that nothing is added when checking the children's scene.
var newPlane = new THREE.Mesh( new THREE.PlaneGeometry( 2000, 2000, 8, 8 ), new THREE.MeshBasicMaterial( { color: 0xffff00, opacity: 0.25 } ) );
newPlane.visible = true;
this.scene.add( newPlane );