Can any one tell me how to Trigger an email when IDEA is promoted to 30 points?

Can any one tell me how to Trigger an email when IDEA is promoted to 30 points?

I am trying to create a trigger that sends an Email Alert (or just an email though Apex code) to the original Idea poster whenever a comment is left for their idea. In looking through the Apex documentation, I noticed it says you can only define triggers for top-level standard objects, but not for standard child objects. When I try to do something like this:

trigger commentEmail on IdeaComment (after insert) {
// send email code here
}

I get "Error: Compile Error: SObject type does not allow triggers: IdeaComment at line 1 column 25". Can anyone point in the right direction to get around this?