What is their reaction when they find something?
Do they show the other children, play with the object, or rebury it?
Leave a small part of the object sticking out with very young children. Can they tell what it is?
Do this with any of the other substances listed for sensory table, such as shaving cream or confetti.
Give children sifters to help them find things.
This is a cognitive (object permanence) activity. Children learn that something still exists when they can't see it, and they have to create a mental image of it.