Construction sand will work their ready for a bath.I provide mine with sand they love relaxing on the beach.Its cool they stretch out & you think their dying.
I usually get play sand in the summer because it's easy to find. I think construction sand would not be as 'smooth', which is fine. They'll eat it like grit so the rougher the better. You can start them whenever you like, usually they start on their own with their food or bedding anyway