It's always a bad idea to leave even shallow wading pools filled with water. They are an irresistable attraction to kids and a perfect example of a drowning hazard. Unless you have an alarm on the doors that goes on when they are opened, ALL THE TIME, or you have a self-latching safety fence around the pool, (yeah right) your should plan on draining and refilling with each use. Children will always be the more precious resource, not the water.