I made a silicone candy mold using Lego bricks. You don't need to make a custom mold. There are many commercially available silicone molds, or you could just skip the mold completely, and cut them with cookie cutters, a knife, or even scissors.
THIS PROJECT IS NOT ENDORSED BY LEGO. DON'T MAKE THE MOLD, OR THE CANDIES FOR SALE. - FOR PERSONAL DELICIOUS GEEKY BLISS ONLY
First I'll tell you how to make the candy.
I'll explain in detail how I made the mold in step 4. It really is very easy. User Gabebillings made the mold and documented the process in photos. See the first comment under step 4 to read his experience and see his photos.
User ibeschieru made a "making of" video showing his kids creating Lego shaped gummy candies! http://vimeo.com/36217289
I bought the Food Grade Silicone from Douglas & Sturgess. They are a great San Francisco store that sells artists materials, mostly for sculpture and casting. They have some great info on their site.
If you have kiddos that might mistake real bricks for the candy, you should consider whether it is safe to make these for them.
Vegan option There is another kind of jelly candy called gellies or jellies. Those are made with Pectin. It's a different recipe entirely. I don't know how well they would mold though.
Here is a Pectin Fruit Jelly Instructable. I haven't tested it in the mold but it seems like it should work. Pates de Fruit