Perfect Substrate for baby Hermanns?

[John Stamos]

I have 2 baby Hermanns under 1yr old and I was wondering what the perfect substrate for these guys would be. At the moment I have 100% Cypress mulch and they love to burrow way down into the stuff during the day to nap and at night to sleep. Someone told me that I should be using a mixture of topsoil and mulch. I'm not sure what's ideal for these guys. Any ideas?

I love my baby C.B. Hermann Tortoise!
(Shell-E)