Mix grass seed in with potting soil. Fill up a nylon. Tie into segments with string to make a 'caterpillar." Soak in water till saturated. Then let the kids spray it with water every day after and watch it grow. When the grass gets "too long" let the kids give it a "haircut."
Try this science experiment with children over the course of several days. Fill up 4 vases with different coloured water and put 1 white carnation in each one. Watch the flowers over the next four days change to the colour of the water.