This is a collection of ideas of what I want to do with my kids when they're older, so I don't forget.
Prepare two plates with agar. After playing outside, let them put their hand in one plate before washing hands. Thoroughly wash hands. Let them touch the other plate. Put ceran wrap over both plates and incubate. See if there's a difference.
Take old jam jar, make hole in the top, approximately 1cm in diameter. Put about 10ml of Ethanol (rubbing alcohol) in the bottom. Place jar in waterbath (when it works, jar will eventually burst, need to not spill burning alcohol everywhere). Close jar, light at hole. See if can get to run continuously.
Two tin cans, Nylon string, build a can phone.