Read over the commercial brief. This is a paper that you should get from your agent that describes what the casting director is looking for. You may get a full description of the commercial or just a brief description of the character. For example, the description may say that the casting director is looking for an athletic child or a studious child.
Dress your child appropriately. Choose clothes that support the role but also look natural. If the casting director is looking for a smart child, you may want to dress him in khaki pants and a button-down shirt. If he's looking for an average child, stick to jeans and a polo shirt. Whether you're dressing a boy or a girl, use bright colors and avoid busy patterns.
Practice the type of role your child might have. You may or may not get a copy of the script ahead of time. Nonetheless, you can practice the types of things that he may encounter in the commercial. You may know that he's going to fall down and cry, so you can practice making scared faces; you may know that it's a commercial for a birthday party, so he can get excited about being the birthday boy.
Put him to bed early the night before the audition. A child that hasn't had enough sleep can get grumpy. It's essential for him to have a proper night's rest in order to give his all the next day. This is especially true if your child is very young.
Eat before the audition. If your child is hungry during the audition, he will be distracted and will not do his best. Eat a meal before you leave the house and keep some healthy snacks in your bag--you never know how long you'll be waiting.