There's really no reason to check for chlorine loss if your SWCG is running through the night--the whole point is to determine if something is consuming your chlorine, or if it's not being generated--and if it's running overnight, then any difference in the two readings is pretty much meaningless. If you need to shock the pool, you need to shock it and hold it there until you're not losing chlorine overnight. Could be one night, could be more, it just depends.

The kids should wait until you're finished shocking (no longer losing chlorine to anything other than sunlight) and until the chlorine levels have come down to an adequate level in your pool, which for a CYA of 48 should be at least 3 ppm before swimming. If you have something in the water that is creating a chlorine demand, then there's no chlorine left over to kill the viruses and bacteria that we're trying to protect the swimmers from!