Dog Kennel Help
 

I just put together a 10x10 dog kennel with a raised plywood flooring. I left a section open for the dogs to go potty, but I don't want it to just be the grass because I know they will end up digging and it will end up being a mud fest. What is recommended to fill in an area like that - gravel, mulch, etc? The most important thing to me is that the dogs will be safe, so I want to make sure it is something that they won't end up swallowing or something that will lead to unhealthy (non-sanitary) conditions. I just don't know anyone with experience with this, so any info would be very much appreciated.

Thanks