Cats and rubber mulch

Hi there,

I have 3 kitties at home and I recently built them an outdoor cat
enclosure (with a cat-door leading back inside my house). They love the
enclosure, but two of my cats eat the grass growing inside the enclosure
- which makes them sick later. So I'm thinking of getting rid of the
grass inside the enclosure. I don't want it to just be mud & dirt
inside the enclosure, so I was thinking about putting down a thin layer
of a long-lasting shredded rubber mulch used on children's playgrounds
nowadays. "NuPlay" seems to be a popular one
(http://www.rubberificmulch.com/nuplay/).

Does anyone know if these products are safe for cats...or if cats will
tend to eat this type of mulch?

Thanks!
Brad