The problem with this is that you get those heartwarming stories about people who's first experience with fish ever was a goldfish they won at the fair, they put them in a 1 gallon bowl, then upgraded to a 10 gallon...and lo and behold 10 years later they're in a 55 gallon tank, happy as can be. I am sure there are members like this in this very forum, actually.
Goldfish as prizes is a tradition, I'm afraid, so the people who set it up I'm sure are going to be resistant to changing. In the UK, of course, it's illegal - just like feeder fish are illegal - they are much smarter about these things.
What I would do is make a petition, and get others to sign it. Talk to the parents and let them know that the excitement of winning a goldfish at the fair is negated by the sadness of having their pet die in a bowl or tiny tank. The longer the goldfish lives, the sadder it will be for them.
I know of a parent who tried, but couldn't, change the minds of the local church organization which hosted the carnival. So what she did was print out a bunch of coupons for $10 off a $50 purchase at Petsmart and a bunch of one page care sheets. Not sure how well it worked - but a great idea, I thought.
Unfortunately, feeder fish are just doomed to be exploited...what really worries me is that it's such a poor lesson for the kids. Grownups should know better! Animals as prizes is just ethically wrong in so many ways.
There has to be an alternative...but what?