Charlotte's Web is a great book which can be enjoyed by people of all ages. The fair can certainly be described as a rat's paradise.
Let's break down what we know about both. Rats are creatures that eat almost anything, and are known for digging through garbage and leftovers for food. A rat frequently is considered sneaky.
Paradise is the ultimate utopia where you can have everything you want, a perfect place.
Therefore, since the fair has lots of people, eating lots of food and thus creating a great deal of garbage, a rat would be in paradise there. A rat would have a seemingly unending supply of food. Additonally, with all the animals being fed, a rat would have an easy time stealing a small bit.