One of the simplest experiments that can be used with kids is the "Garlic in the Sock" test. I have mentioned this on the air and teachers that have tried it all reported success.
The experiment is to have each person put one peeled clove of garlic in his/her sock under the foot or in the toe area. People without socks can do the same thing I guess. It won't take long for the taste of garlic to be distinctive in the mouths of the participants.
The point is to show that chemicals, natural or toxic, can move systemically through the body by simple skin contact. As apposed to natural things like garlic, toxins move with the same or even better efficiency. The bottom line is that running around barefooted on grass treated by chemical lawn care companies is a good way to get those chemicals moving through the body and leading to illness. It's reason enough to go organic.