I used to keep a lot of sand and gravel around so when you need to wash it, you can use 1/2 c bleach in 2 gallons of water in a sprayer (like the pump and spray for insecticides) then give it a good rinse with a hose. The bleach shouldn't harm them, give the "pool" a good rinse, that is frequently one of the culprits for yuck smell.
Just make sure you rinse well.
I have recently discovered two things that helped my odor situation.
1. DE! (Diatomaceous Earth) I sprinkle it on the poo and it dries out immediately and NO Flies.
2. I changed feed! Someone on here was discussing DuMor feed (TSC) and how there was so much more poo from it. They were absolutely Spot On! I changed to a layer from a local grain mill and was absolutely amazed at the poo reduction. Less poo, less opportunity for smell, less DE, and fewer pine chips.