Question if you have a digging pit - what do you use? sand, mulch, etc.?

We're having the entire back yard landscaped with a big part of it designed as a playground for our puppies. They looooove jumping over hurdles (ditches, fallen trees etc.) so I've put a little hurdle course of four logs there for them to have fun jumping over. We're going to get some agility equipment too, just for them to play in.

Since they're puppies, they have a blast digging. I want to create a digging area for them where they can get out all their digging urges (rather than trying to uproot one of the 100-year-old oaks on our property ).

I'm thinking of building maybe a 6'x6' square, boxed in with some pressure-treated lumber. I know a lot of people recommend sand, but I can just picture my pups constantly needing to be hosed down from all the sand in their curly fur.

Has anyone used something like cypress mulch for their pets' digging area? any pitfalls to using mulch?

Of course I'll spend time training them to dig there (with positive reinforcement, burying toys and treats, and "leave it" when they start to dig in the dirt)... any other tips would be really appreciated... especially tips to make it as fun as possible for my sweet doggies!