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Total time: about 60 minutes.
Cast replicas of fossils are common in museums. Sometimes there are just too few specimens available - it would be impossible for the public to see them were cast replicas not used. On other cases, the real fossil may be is too fragile. Maybe the real ones are just too darn heavy. For whatever reason, the use of cast replicas is common and legitimate. Liz Martin-Silverstone, wrote an excellent post on the legitimacy of cast replicas if you want to learn more about why they are so important. Learning how to make some basic cast replicas is a great activity for the kids (and you!) and gives them another experience that mimics an aspect of real paleontology.
What you'll need
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What to do
Creating a cast replica involves two phases. First you will need to create the rubber mold (phase A) and then create the resin cast (phase B). When the resin hardens (takes 10 to 15 minutes) you can paint the fossil replica if you wish.
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A. Create the mold
A.1. Clean the fossil - this might be the hardest part - you need to get all the dirt and loose rock from the fossil without damaging it.
The following steps require a mold putty, I used the Alumilite kit but any putty kit designed for making resin casts will work. Instructions are below, but consult the instructions that came with the mold kit. A.2. Take equal amounts of the white and yellow putty - enough so that the combined amount of putty will cover the fossil without getting too thin. A.3. Combine the two lumps of putty into one and mush and squish them together until it is an even color - a light yellow.
A.4. Cover your fossil with the putty, leave an opening to get the fossil out. The putty will turn into a rubber and will stretch, so it doesn't need to be a huge opening, but it does need to be big enough to pour the resin solution into (phase B).
A.5. Wait about 20 minutes - you'll know when the molds are done when they are nice and rubbery.
A.6. Slowly and carefully work the fossil out of the mold - it may take a bit of effort. 
Move on to phase B.
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B. Make the replica
The following steps require a resin solution. I used the Alumilite kit but any resin kit will work. Instructions are below, but consult the instructions that came with the mold kit.
B.1. Make sure you can prop your mold up somewhere so that the opening is facing up. Some play-dough or clay works. Or even a disposable cup for larger molds. 
B.2. The kit should come with two solutions and measuring cups. Pour equal amounts of each solution into separate measuring cups - enough so the combined amount will fill the mold you just made.
B.3. Mix the two resin solutions in a disposable cup and stir them (they should come with stir sticks, if not a popsicle stick or some other DISPOSABLE stir stick or spoon works. DO NOT GET THIS STUFF ON YOUR FINGERS OR FURNITURE.
B.4. Once stirred, don't let it sit. Pour it carefully into the mold. B.5. Once poured, let it sit for about 10 to 15 minutes minutes (consult the curing time on the kit's instructions). The cured resin will turn into a solid white. 
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Here are some photos of casts (right) we made along side their original fossils (left). From left to right - spinosaurus, crow shark, trilobite.
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Finally - if you want to make your case more realistic. Paint it!. Use rock/earth tones- grays and browns. It doesn't need to be a uniform color...few rocks are! Here is the originals (left) and the painted replicas (right) - from left to right - spinosaurus, crow shark tooth, trilobite
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