Tell your child the good news with genuine enthusiasm. Show your child that you are happy for them and celebrate their success.
Discourage boasting or bragging. Real winners don't make other people feel bad when they succeed. Encourage your child to be humble about her success.
Keep all comments positive. Don't bad mouth or allow your child to bad mouth the other child actors and actresses who tried out for this role.
Instill a sense of self esteem by allowing your child to know that you still love him even if he had not been casted. Explain that you love your child for who he is and while you're excited by his success, you would still love him just as much without it.
Talk about the responsibilities that this might entail such as long rehearsals, missing school or related issues.