Fun time happy garden explosion has a great idea of how to make what they call a guerrilla garden. It does require some unique ingredients though including worm casting, which can probably be bought at any gardening store. Basically you take all of the ingredients and mix them together to make the "bombs." Your kids will love playing with the consistency. Once made you literally throw them into an empty plot of land and watch them grow. A fun way to spruce up the neighborhood and show your children cause and effect.
Good for ages 3 and up.
Get the instructions here:
http://funtimehappygardenexplosion.blogspot.com/2007/08/guerilla-gardening-seed-bombs.html
Gardenista uses clay powder and compost.
Get the instructions here: