Once your kid begins going out on acting auditions for kids, keep in mind the tendencies of kids. Some days will be good days; some will be tired days; some will be high-energy days; and some days when, no matter what you promise, they won't do anything for anybody. Will the audition people understand? Of course they will.
There will be other auditions for kids, other kid acting jobs, and there's no need to burn bridges with casting directors over an attitude caused by a missed nap.
A casting director wants to see a kid act like a kid. When reviewing a part, besides specific character traits that will be noted both by the content or the casting agency, kid actors should be themselves. No casting director wants to see an eight-year-old act like a twenty-five-year-old.
The only thing your child really needs to know is to have fun!
When it's your child's turn to audition, tell them good luck and wait outside. A casting director needs to know that a child can perform without the parents around.