I recommend getting an engraved pet tag. If you go to a pets-r-us type place, they often have an automated tag making machine. Simply use your childs name, your name, and your cell phone number. Then lace it through the bottom of your childs shoes. If you become lost, most of the CMs will notice the tag and can take appropriate action. Obviously, you'll need to carry your cell phone (and I would assume you have a nationwide plan).
Secondly, if you have a camera phone or digital camera, take a picture of your child each day before you head off to the park. Carry your phone or camera with you. This way if you do become lost, you can quickly refer to what your child was wearing for a discription -- trust me, in a stressful situation such as that you're likely to forget that day's wardrobe choices! This way the CM's can put out an accurate APB which should help speed the reunion.
Servants of Evan