You only need a model release if you're going to use the shots for a commercial use. If you're just shooting a family portrait, for the family, then you don't need one. If you're going to use the images in your advertising, then you should get releases. What I tend to do is if I'm going to use the photos for something, I'll ask the client when I deliver the job, and then get the release.
Also, you need to get releases from everybody in the shot, unless it's a large crowd. If there are minors in the shot, then their guardians have to sign them.
David Buzzard's Technical Blog