dee, If they want to make concrete mushrooms just buy bags of ready mix concrete mix. These bags of concrete mix have portland cement, sand and gravel already mixed.
If they want to make a mortar mushroom use one part portland cement to 2 parts coarse sand.
You can use plastic bowls sprayed with "PAM" baking spray for the mushroom cap. Or make the cap upside down in a damp sand mold. The stem can be made from lots of things like, the cardboard tubes from paper towels to wire armatures.

Thank-you for your help.