What to put in a dig box?

I want to make a digging box for Charlie ... Any ideas what the best type of soil/sand is to use?

I think child safe sand is fine.

That was my first thought but I worried about it getting in their eyes ?

I tried various things and of course some are more appreciated than others.
Sand- no response at all, they just looked at me saying so what?
Soil-got at least sniffed and sat in saying so ok!
Found top litter called Vitakraft County home Birch Grove and mixed that with soil and straw-well they love it and Bubbles works her hardest. She protects her digging box just as much as her food bowls. I guess she would use it as a nest if ever a litter would turn up!  
It is about£3-4 per bag of 600 gr that lasts us two or three fillings.