I'm using Hype as an html5 animation tool for elearning purposes. The main elearning course is built in Adapt and the Hype interactive animations will sit inside Adapt iframes. The idea is that Hype sends an interaction 'complete' message via postMessage to Adapt. The Adapt iframe is ready built to listen for a postMessage sending 'complete'. So the external iframe is the parent and Hype is the child.
If I put the following in a simple html page window.parent.postMessage('complete', '*'); on my web server and reference the html page within the iframe I can trigger the iframe interaction as complete without any issues.