My neighbor's cat thinks my 14 ft x 14 ft sandbox is a giant litterbox for him so we purchased this:
http://www.amazon.com/dp/B002A91E6I/...SIN=B002A91E6I
We bought it from Menard's and you can get it ready to spray or in concentrate so you can mix it yourself. It is safe to use around kids, vegetation and plants as it is made with organic ingredients. It also seems to stay put (even after rainshowers etc) and we have only had to reapply after about 30 days or so.