it's a cute idea, but i had the same problem. As soon as I put in my slightly thickened jello, the cake floated to the top and the jelly beans went to the bottom. So I let it set for a while and figured I would add more jello once the first layer was more firm. I guess I should have waited 6 hours to do that, b/c when I added the next round of slightly thickened jello, the cake sucked that up too! So it looks like a pretty gross mess.
If someone can tell me how to get the cake under control it may have worked, but for now my friend is getting a really weird/gross looking birthday cake.