I made these for a Harry Potter party as well (for adults!) and they were a huge hit. The party was months ago, and they still get mentioned regularly.
I just made regular cupcakes (no Jello) and, as others have mentioned, frosted the cones before putting them on the cupcakes. I spread the Cool Whip on the cones and then rolled them in the colored sugar. It still took a long time and a lot of work (and I only ended up making a dozen of them!), but the rave reviews were worth the effort.
One caveat though: everyone loved how they looked but wasn't all that excited about the taste. The cupcakes were fine, but the cones are really too sweet and are best used just for decoration.
To answer a previous poster's question, the star candies/sprinkles stick to the Cool Whip or frosting. You just kind of push them in to the frosting and they're fine.