Adding a child view to the PDP which adds a SCCollectionView. Is it possible to trigger a function once all cellViewInstance are rendered? Something like this?

this.pdp.cancelableOn('afterShowContent', function () {});

Kinda. You can override the collection view's render() function to call the parent class's render() function and then put code after it

Triggers for each child view

OK, well there might be some trickery you can do where you have a master promise on the parent view and each child view adds a child promise to it; and then once all the child promises are resolved, your code executes. But this is getting rather complicated and wouldn't really be able how to advise doing that in the context of a Backbone SPA