Add a metal shader with transparency to child.

addShaderWithTransparencyToChild.swift

func addShaderWithTransparencyToChild(child:SCNNode) {
  let program = SCNProgram()
  let shaderName = "YOUR_SHADER_NAME";
  program.fragmentFunctionName = shaderName + "SurfaceFragment"
  program.vertexFunctionName = shaderName + "VertexShader"
  program.isOpaque = false;

  // CREATE A MATERIAL, ADD THE PROGRAM, AND ADD IT TO THE CHILD.
  let shaderMaterial = SCNMaterial()
  shaderMaterial.program = program;
  shaderMaterial.transparencyMode = .rgbZero;
  shaderMaterial.blendMode = .alpha;
  shaderMaterial.writesToDepthBuffer = false
  shaderMaterial.readsFromDepthBuffer = false
  shaderMaterial.cullMode = .back

  child.geometry?.materials.removeAll();
  child.geometry?.materials = [shaderMaterial];
}