Take a headshot of your child or have one taken by a professional photographer. Unless your child has extensive experience in the field he's auditioning for, his head shot will be the most important part of the portfolio. Make sure your child is smiling in the headshot.
Take at least two additional pictures of your child: one full body shot and one of your child in a pose that suits her personality. Avoid using any props or brand names in the photograph, as this may limit its marketability.
Gather together the best pictures from any previous photo shoots and add them to the portfolio. If your child has no experience, leave this step out or supplement it with additional photographs of your child in poses.
Arrange the pictures in a folder or portfolio. Lead with the head shot, then follow with the best pictures first. The site, Modeling for Kids, suggests adding a composite card or a sheet with multiple pictures on it from different shoots, to the portfolio if your child has enough material to warrant it.
Add a sheet of paper to the portfolio with your child's name and age on it, as well as any experience he has in acting, modeling, dancing, or any other kind of relevant talent.
Update your child's portfolio every year with new pictures. Because children grow fast, portfolios can become outdated very quickly.