Need Advice on Dig Boxes

[Mother of Eleven]

  I finished the dig box for my ratties, and they love it.  I added some of that grass they sell for cats, and some flat river stones around the edge of the box.  It looked nice.  Half an hour after they discovered it, they had completely covered the grass with dirt, and the stones are not to be seen.  I also spread some grass seed in with the dirt to see if it would grow; they ate all the seeds.  It was great to see them having so much fun.

My question is about the soil.  When it comes out of the bag it has a little moisture in it.  Should I keep adding water to it to keep it moist or just let it stay dry?  The box is pretty good size, 23"X14"X12" with about 6" of dirt in it.

[WS]

Dig boxes are a ton of fun. -- I like the idea of river rocks.   - I've never done anything fancy like that... I've just made dig boxes out of storage containers and sterile reptile soil (bought it in a pet store) with a spice rack for a set of stairs leading to the container. ---- I've heard of people growing things.... such as planting a little bit of ratty seed mix and things sprouting from that.  You can also plant edible plants like wheat grass.  However, part of digging fun is digging up the plants!  
And yes, you do have to keep the soil a little moist if you want things to grow.