I would just stick with opinion as your defense. You are allowed to criticize and comment, and your comment does fall under opinion.
As far as being called a "fag**t", this 9 year old kid may think that implying he was homosexual is defamation per se - he may have had all his 9 year old buddies watching the video, and he may forever be teased by your comment. His mother probably has to deal with her son's deflated self-image, too, especially if he was really excited about being on YouTube. Also, the viewership of YouTube is so great that 30 minutes probably exposed your comment to a heck of a lot of people. It would have been better if you could have removed the comment before the mom, or whoever, did it, but it is good that it has been deleted. I wouldn't stress the length of time your comment appeared, however, unless it comes to mitigating damages.
And, if it DOES get to court, which I honestly don't think it will, don't say you didn't mean to hurt the kid. That was your intent, otherwise you wouldn't have posted something so mean-spirited. Just say you were stupid. THAT is far more believeable.