What about getting a sandbox cover to deter wandering
cats from peeing in the box. They are sold in many places in
the U.S. & probably available where you are too. Like these;
http://www.detailedplay.com/AccRoofSand.htm
Of course it's really the parent's responsibility to do this if it's
their children's sandbox, but I'd do it as an answer to their
complaints. If it's not possible to buy one, I'm pretty sure one
could be built to fit without to much spent on tarp,etc. If it
would help Pjevs, might be worth a try.