Gross your kids out by drizzling colored alginate (liquefied brown seaweed) into a solution of calcium chloride. When the alginate hits the solution, it instantly polymerizes a skin that surrounds the liquid, forming what appears to be small boogers and eggs. Wonderful polymer reaction and a great example of a chemical change.
This is the same lab as Seaweed Caviar and Instant Worms. We just made a few adjustments so you can use it for Halloween!