Outdoor Life magazine just published a video on how to make your own ballistic gelatin. It involves Knox unflavored gelatin available in grocery stores everywhere and a loaf pan. The size shown is large enough for shotgun pellets and rimfire bullets.
This seems like a good project to do with your kids on this long Memorial Day weekend. It will be something you can do together and it will help emphasize the need for safety when handling a firearm.
Doing it yourself is certainly cheaper than buying pre-made ballistic gelatin. Prices I saw of pre-made blocks that meet the FBI 10% gelatin protocol start at over $100. Compare this to the price of a pound of Knox unflavored gelatin on Amazon which goes for $14.90.