If you don't do it, people at the pageant will. They will get a
spray tan somewhere, you might as well be the one to get paid for it.
I've sprayed several pageant girls, and have had very little problems. I always let the parent know that if the child seems scared, or upset, I won't continue the spray. Usually 6 and up are ok. I had the misfortune of spraying a 3 year old (almost 4) once and have decided that I won't do that again. She could not be still and she ended up ruining the spray by sucking on her lower lip and drooling when she drank something after the spray.
I don't approve of the whole child pageant thing, but it's my job to provide a service, not judge a client wanting the service. That's how I look at it.