I came across the recipe at Our Best Bites and couldn't resist giving it a go. The result was pretty wiggly squares that tasted like Skittles. :O) The only thing you really need to make sure you have plenty of is time. Don't wait until the last minute to put this together since it requires two rounds of gelling.
All you need is Jell-O, unflavored gelatin and sweetened condensed milk. And because Jell-O comes in a variety of colors you can use whatever colors fit your theme. The recipe linked above makes a big 9x13 pan, but you could easily cut the recipe in half and use a 9x9 pan.
Give it a try for a fun treat kids and adults alike will love!!