The pool chlorine should have dose rates on the packaging, but you should double or triple dose it to make sure there is nothing alive on the rocks. You can put a lid on the container too to help reduce the fumes but be careful and hold your breath when you lift the lid so you don't get a lung full of chlorine gas.
If the sand is an inch thick then stirring it up several times should allow the salt water to get in. If you have 3-4 inches of sand, then the bottom of the sand might not get salt water. In which case you could remove it and put it in plastic storage containers with salt water. Have half an inch of sand in each container and let it soak for a bit. Then rinse and dry in the sun. Once it has been salted, rinsed and dried, there won't be anything alive in it.