Wait a couple of hours then lift the sheets. The earthworms will have come to the surface because of the moisture. Give your gardeners paper cups and have them collect as many earthworms as they can find. Add the worms to their garden, put some in yours too. Earthworms help aerate the soil.
Children get very excited when they see things grow.
Try this indoor seed-starting project. Take the lid off of a glass jar and line it with a damp paper towel. Put a few zucchini seeds in between the glass and the towel. Replace the lid and put the jar somewhere safe, like on the kitchen counter. Make sure to keep the towel moist and in a few days have the child check the jar.