Mushrooms are fungi. There are thousands of different varieties in the world, not all of them edible. As many are poisonous, remind children that it isn't advisable to eat or handle mushrooms they find in the wild.
One method of identifying different varieties is to reveal the spores.
Spores are cells that are use for reproduction. Thousands of them appear on the gills, found on the underside of the cap. Once the spores are mature they are ejected from the mushroom. They are so small that the slightest breeze carries them away.
A great investigation for children is to take a spore print from a mushroom. Use shop-bought fresh mushrooms. Do not use mushrooms from the wild as they may be poisonous.
Mushroom spores are different colours, the most common being white or brown. If you don't know what colour to expect, do two experiments - Jone using white paper, the other using dark coloured paper.
Remove the stem and observe the underside of the mushroom. Make sure that the gills are exposed, and cut away any of the cap that is in the way.
Place the cap on the paper, with the gills face-down. Place a glass bowl or jar over the cap to stop the spores getting blown away. (In order to make a spore print the cap must be fresh - if it is getting dry put a couple of drops of water on the cap before placing the bowl over it.) Leave overnight, then carefully lift the jar and remove the mushroom cap. With luck, there will be a spore print on the paper.
If you have a microscope, use it to examine the print. Fix the print in place by spraying it with hair spray or the adhesive sprays used to fix pastel drawings.
The Australian National Botanic Gardens website contains lots of information about fungi, with examples of spore prints and tips on how to make them. www.anbg.gov.aufungiindex.html