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
Step 1: Tools and Ingredients
I made them in small batches so I could do many experiments.
The recipe is for a small batch (one full mold for me). You will probably want to double, triple, or quadruple it.
Recipe:1 package of flavored Gelatin (Jello)
3 packets of Unflavored Gelatin (Knox is most common in the US - they come 4 packets to a box)
1 500 mg Vitamin C (optional but it adds some great sour flavor)
1/3 to 1/2 cup of water
Tools:Pyrex Measuring cup
A cooking pot that the cup will fit into
Some kind of spatula.
Mold or a flat bottom pan to pour the candy into
Mortar and Pestle (or a hammer?) to grind up the Vitamin C
In the photos you see me using a shallow pan with just about an 1" - 1 1/2" of water to melt the syrup. You should go ahead and use a pot with higher sides, so you can get the water level up another inch or two.
I used the shallow pan so it would be easier to see what was happening in the photos. I use a sauce pan when I make them now. A double boiler would be ideal, but I couldn't find one in the stores I visited. They aren't as popular as they used to be.
The syringe I used to fill the mold in the photos is optional. You can just pour the candy in. I bought the syringe at Tap Plastics. You can get big syringes like this in cooking stores also. There are syringes for injecting stuff into meats. They can handle the hot syrup really well but have a very small hole at the tip.
I have a metal and plastic trigger activated frosting gun. It is used to decorate cakes. I don't remember when, or where I bought it. I tend to buy tools when they are on sale, and think up a use later. I decided to try it out with the gummy syrup and it worked great. That is what I use now.
(The wire strainer in the photos is not needed. I reviewed my notes on the experiments and the clearest candies - the green and red, didn't use the strainer.)