Print out several copies of the script. Read them carefully and highlight your child's part.
Read the scripts with the child until she knows her lines. If the child can read, have them memorize the lines anyway.
Read with the child until she is comfortable with the lines and seems to have fun with the process. If you force the child into it because you want them to be in show business, it will show in the audition. Encourage the child to relax and be himself even while he's reading lines for another person.
Prepare your child's clothes early and have the child wear them before the audition. Have them wear a shirt or blouse with no logo. A polo shirt with a pair of jeans or khaki pants works well for the boys. An unpatterned dress or collared blouse and pants work well for girls. Do not dress for the character.
Plan a travel schedule so you get to the audition well ahead of the scheduled time. It will not help the child to be rushed into the audition because you are running late.