Well, sand, pebbles, and gravel are not recomended substrates for frogs/toads. The risk of gastrointestinal impaction is too great and can lead to death. Better substrates are ground coconut shell (often sold under names like eco-earth, bed-a-beast, and so on) and sphagnum moss. They hold moisture fairly well and the frog/toad can bury itself in it with no problem. They can be spot cleaned and when it gets really soiled, they can be easily replaced.