I've got my product MVP out, and I want to test it and get feedback. Ideally I'd like dozens of people in my target market (for this example, kids into games and fantasy books) to test it. I've cold and warm emailed librarians and mailing lists and gotten some good responses, but just a trickle - one or two who are hard to stay in touch with and are sporadic.
I would be willing to pay money to have an event where kids spend a good amount of time with our product, but it seems that even money can't buy an audience. How can I find reliable, convenient testers?
To answer the reliable and convenient testers part of your question. I can recommend a service like bugfinders.com. They have an army of testers who pull apart your system and give feedback to meet your specification.
As for your demographic, generally they aren't the paid sort of audience so you will need to find hangouts where they can be found or can find you.