Regular household bleach will do the job if you use it with care and water rinse it properly.

Mix the bleach at one part bleach to 20 parts water ratio. One ounce bleach to 20 oz of water. Use a pump sprayer to wet down the stone area. Let sit for 30 minutes, and then rinse well with your hose.

Keep the dogs off of it until you rinse it well.

Or you can order a kennel sanitizer from one of the pet supply houses....

Jeffers Pet Supply or Revival Animal Health are 2 that I use. They have several different types that are mixed with water and will do the job as well.

One caution: do NOT use the bleach any stronger than a 1:20 solution, and remember to let it sit, and rinse well. If you leave too much, make it too strong, or don't rinse well, it can give doggies sore feet for a day or two.