Here are some tips that can help your child prepare for modeling auditions:

You'll usually be handed a script or told what to do on set a few minutes before shooting. Help your child memorize the script and prepare him for such a scenario in advance.
Help your child memorize the script

1. Don't stress too much about what your child will wear to the audition. It is not the clothes your child wears that matter during the audition rather it is your child's attitude, confidence, and presence in front of the camera that is going to make all the difference.
2. Teach your child camera etiquette. Show him how to stand confidently and face the camera and also how to take instructions from the director.
3. Don't stress yourself or your child out, and try to make auditioning a fun experience. A kid who is not forced to do anything and who is happy is definitely going to have a blast in front of the camera.
4. Make sure your child is well-rested, positive, and not irritable before the audition. Also, if at any time you think your child will not be able to audition, notify your agency well in advance.