Tardigrades (or water bears, moss piglets) are fascinating cute little creatures, and they are quite heat and cold resistant as well. Kids would love to look at them (and adults!). Isolating them is not too difficult. Take some moss, and soak it with a bit of water. Wring out the moss and collect the water for microscopic analysis. The tardigrades (if they were present at all) were also washed out and can then be observed.
DIC micrograph of a water bear (Credit: CC-BY-SA-3.0-de by Frank Fox (www.mikro-foto.de via Wikimedia Commons).