Why not go out and use something relatively safe to knock all the conkers out of the tree yourself and leave them in a bag on the front, that way you can make sure your cars are out of the way. I know you've probably got better things to do, but you will be etched on the minds of these kids, who later in life will tell wonderful stories to their grandchildren about this wonderful lady who would leave conkers for them on the way home from school. Not convinced? Naa I wouldn't bother either. You could though ask a couple of reliable sensible kids on a weekend to work out how to get all the conkers down. Once the rougher ones look up and don't see any they'll stop throwing stuff. Good Luck