That's a great idea but you could add some more kiddie involvement without adding other elements to the party.

Isn't it possible to make chalk 'sculptures' using just water and chalk? Maybe the kids could have a workshop where they make little Silburys, standing stones and stone circles using chalk and water paste in paper mache moulds, the sculptures could then be placed inside Silbury where nature will take it's course. That way there's lots more involvement for the kids (got to be a good idea and plenty of 'touchy-feely' for the suits) and they could have their piccie in the paper with their little sculptures.

Of course, there's also the problems of influencing the authorities, don't know what it's like over there but over here if you don't foam at the mouth at objectors and call them tree-hugging-eco-warrior-marxist-anarchists then your opinion is off the spectrum.