Digging box

I want to make my bunnies a larger digging box, should I use earth or child safe sand?

They are long haired and light coloured, I'd like something they could use outside so maybe sand is be as it wouldn't be as horrid when wet?

In my experience, bunnies prefer to dig and flop in soil BUT sand is a great alternative. Just don't use builders sharp sand as it'll stain any white fur orange/brown. I'd use silver kiln dried sand which I think is what's used for sand pits.