Wow, that would be difficult to pull off ... for me, at least.
The timing -- that alone --- would be a tough call.
I think I might try having them hatch the 2-3 days before the fair opens and then transporting my hen and chicks and setting them up at around 10:30pm at the fairgrounds the night before the fair opens, you know, so that the hen doesn't notice the move very much and can settle down in the dark with her chicks for a few hours before it gets light. And then I'd probably sleep in my vehicle and go check on them first thing in the morning to make sure the hen was acting properly. She could get confused and start pecking her chicks. If there's a problem, then you could perform a rescue before the fair opened for the day and scratch the plan before any visitors appeared on the scene.
I mean if they were set to hatch during the fair, the mom won't even take them off the nest until day 2-3, right? That's how mine work. It depends on if the last one hatches during the day or evening hours. So, if you take them to the fair (on its first day) two days after they've all hatched, then they can be viewed by the visitors. Plus there would be less chance of the hen getting distracted or upset.
Will you have a large pen on the ground for them or will it be a cage on a table? I guess the fencing will have to have some small holes. A few of my chicks can get through chicken wire when they're first hatched and the chick and mom get all anxious and squawky and chirpy.
I think I would initially set the broody hen or hens (not sure how many you might take to the fair) with the eggs in portable boxes at home that I could transport all at once to the fairgrounds. That same box would be put in the enclosure for the hen and chicks. That jostling in the vehicle on the way to the fairgrounds, though, might bother the hen. I know I can walk a hen from one coop to another (taking all of 2 minutes) right after she's hatched a batch of chicks and put her in a different coop/run combo pen and have everything be great in the morning, but travelling in a car a few miles might be a little bit too much. But you'll never know until you try.