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."
The cord I use is twisted nylon (usually C-lon or Superlon brand) and is available in four different sizes. I favor the middle of the road size, which is .5mm in diameter. It will fit through an 11/0 seed bead and an 8/0 bead will accommodate two cords. Fiber in Jewelry: Micro Macrame