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."
This looks so inviting. I would imagine this provocation could be presented during an investigation on weather or natural disasters or even as an invitation to explore patterns in nature. How else could you extend this provocation?