I've solved it. needs to override ContentService_Saved where we check if the content service is from the required content type to add another content service as child. This means in Umbraco 8 need to do the following:

[RuntimeLevel(MinLevel = RuntimeLevel.Run)]
public class MyComposer : IUserComposer
{
    public void Compose(Composition composition)
    {
        //composition.SetContentLastChanceFinder<My404ContentFinder>();

        composition.Components().Append<MyComponent>();
    }
}

class MyComponent : IComponent
{
    public void Initialize()
    {
        ContentService.Saved += this.ContentService_Saved;
    }

    public void Terminate()
    {
    }
    private void ContentService_Saved(IContentService sender, SaveEventArgs<IContent> e)
    {
        long totalChildren;
        var contentService = sender;
        var ents = e.SavedEntities.Where(x => x.ContentType.Alias == "myPage");

        foreach (var node in ents)
        {

            if (!contentService.GetPagedChildren(node.Id, 0, 10, out totalChildren).Any(x => x.ContentType.Alias == "ComponentFolder"))
            {
                var pageComponentsFolder = contentService.CreateContent("Components", node.GetUdi(), "ComponentFolder");
                contentService.SaveAndPublish(pageComponentsFolder);

            }
       }
    }