Putting together a kid's competition hunt should be pretty easy, as long as you think ahead about what not to do. I read a story (may have been here on Tnet) about someone putting together a competition hunt and making it a real challenge. A guy seeded a park with about 90 nickels. We know, of course, that nickels are the coins most difficult to differentiate from trash. That would have been a bit of a challenge. What he did, though, was bury all of the target coins standing on edge. By the end of the hunt, a bunch of nickels had been found, but (if I recall correctly) only three of the nickels that were planted. He simply made the hunt too hard! How would you find those nickels? You'd have to strip the park of most of its trash (devastating the ground in the process) to get them.
So, I would recommend using dimes. It's a good idea to use some type of standard target and coins are wonderfully uniform. It's also good that most detector manufacturers would have likely tested their units against dimes at some point. So, everyone should be able to hit on them, they are quite conductive (separable from trash), and there are relatively fewer of them. Pennies are everywhere (people often just don't bother picking them up). Dimes are less common. If you seeded a park with 100 pennies, there may be 1,000 pennies to find. If you seeded a park with 100 dimes, there may only be 200 dimes to find. If they're all buried flat, it should make for an easy, all-about-the-fundamentals kind of hunt.
If anyone can help you fund it, then you could do the coin thing and also bury some special prizes a few days ahead of time (taking careful note of where you buried them, of course, in case they are not found). You could bury a small silver band (ring), a small gold band, and a larger gold band. If you find enough stuff yourself, you wouldn't even have to buy the prizes. It's a lot cheaper to find them than to buy them, and they feel a lot more "authentic." The kids that find the gold and silver won't soon forget it!
