In our years of breeding crickets we have found that adding substrate to your cricket keeper is unnecessary and just creates more work come cleaning time. If you really feel you need to use some sort of substrate than we would recommend using rice or vermiculite as this will also help to reduce moisture in your container. You will find that your crickets, especially if you have a large amount, will leave behind a surprising amount of poo for such small creatures*.
As long as it stays dry, the poo will remain in a powder type form and is fine for your crickets to be exposed to. If the poo gets wet it will quickly begin to smell and can cause other problems in your your container (such as ruining your egg cartons) and should be cleaned out as soon as possible.
*DID YOU KNOW: If you make sure your cricket poo stays dry and you collect it, it is a great fertilizer for plants and lawns!