Copyright/privacy law is very complex in the U.S. While I do agree, that you never can be too cautious, it would be very difficult to lose this case if you got sued over it, to the extent that you would have to pay damages anyway. The first reasonable thing would be that you are asked to remove the photo, and if asked you should comply unless it was taken in a public space, then you'd have a big argument. Even if not, you could still argue reasonable expectation of privacy. You need to think about copyright/privacy laws in more than two dimensions, meaning one thing is lawful and the other is unlawful. There is a grey area, where you might be bending the rules without seeing any ramifications. I'm sure somebody is going to give me crap about this, but it is the truth. You need to consider a couple things: Did the parent know about and allow the photo in the first place? Did the parent know it was being published on the web, or being viewed publicly in some way? Lastly, is the photo so good that it really exemplifies your style to the extent of using it on your own site? You'll have to answer that.
Also, just because your running a business doesn't necessarily mean that you're not an artist as well, and while a portfolio is often considered advertising by photographers (even online), its possible it could be viewed by a judge/court as editorial, with more to back this up if the photo was in a public space of any kind where there was no reasonable expectation of privacy. Obviously you weren't inside the home of the parent where some sort of privacy would be expected, sounds like it was at a party.
Here's some really good concise info on the subject:
http://www.danheller.com/model-release.html
Dan Heller's "Chapter" is brief but it shows just how grey the area can be, you won't find more info without taking a course on it and several books.
Again, to back up my statements, I have a degree in Advertising, which required two years of Media Law, which is a less specific term for copyright/privacy/First Amendment law. If anyone has access to a course in Media Law at your local university I suggest you take it if you have any interest in doing photography. I promise it will be one of the hardest classes you've ever taken, assuming the professor is worth his salt.
I should also say, I use photos I take in my porfolio without model releases. The key term is "reasonable expectation of privacy," was it expected or not? I've sold thousands of prints online and never had a parent ask me to remove a photo of their child from either a password protected gallery or my publicly viewable portfolio section.
