Now there are multiple ways to find and audition, under normal circumstances Disney has talent scouts that are out at different events that try to find talent of which they can set up for interviews and auditions with the larger corporation at hand. Another method is to seek a talent scout yourself for an audition. If you meet their standards they would be more than willing to help you get an audition to progress further up the ladder. Also be sure to have a good resume on hand when you go to your interview along with a cover letter and headshot. These are essentially so that Disney can keep you application straight and as a quick reference for directors who will be reviewing your audition.
Also be sure to get there early, I would err on the side of caution and show up at least an hour early or more. It may sound extreme but it is as small price to pay considering the alternative would be that you miss your audition and therefore do not get the role. I say this because as always there could be unforeseen circumstances and delays, these are common with auditions and it is a good idea to plan ahead for such possibilities. However also remember the fact that your child may not be able to remember all the lines that he/she has been assigned and may need to read from a script. If this is the case be sure that your child can ready effectively but also does not shout when they read the script, they only need to project they voice enough so that they can hear well. Also having them get a good night's sleep beforehand is also a good tip to.
Now, take your time and put you all into your child's audition. Also children do not nessicarily handle their energy as well as adults do. They may have high and low points during their audition and it does nothing to scream or to get angry at them. Just let them go through their audition at a rate and speed that they're comfortable with. The director understands this fact as well so just let your child have fun with their audition.