I rented a space for $20 and brought my table and manned it myself.
Especially if it's your first tack swap, I think this would be a good route to go. It minimizes your work. In Idaho, we have "Idaho's Largest Garage Sale" that's run this way. It's at the fairgrounds - if you have stuff to sell, you rent the number of parking spaces you want to set up in (I believe it's $30/space for regular people, and $100/space for commercial sellers - Of course, they can definitely justify charging that because thousands of people show up, many lining up before the thing even opens!). It's one of those lots with numbers on every space, so it's pretty convenient to find your space and set up all your own stuff in your own space. Because this is such a big deal, there are a lot of rules about when your space has to be set up, what time you have to stay until, etc., but since yours will likely not be as huge, you could probably be a little more flexible. However, there is the added benefit that, as a shopper, I know everyone will be there with all there stuff when I go, so I try to get there before anyone else!
Anyway, all your leg work is done BEFORE the event - all you have to do the day of is manage and make sure things are running smoothly. You also avoid people getting upset with you since you were responsible for their personal belongings and the prices you took for it. I would NOT want to get into that mess, especially with 4-H kids involved!
One additional option you could do is to combine this with Joe4d's idea - you could have one area set up where the kids could sell any tack people donated as well as take donations from people coming to buy stuff. That would be way fun for them! Just give them a minimum price for each item and let them have at it! That would be a great learning experience for them as well. Just make sure there's an adult nearby to back them up if someone decides to be a jerk.