I recently found this website (MabelsLabels.com) which offers a product called "My 411 Wristbands". These disposable wristbands can be customized to relay important information about your child.
“My 411 Wristbands” are designed to give parents peace of mind when dropping off children at birthday parties or field trips, or even when taking them to amusement parks. Other adults or caregivers have a means to contact the child’s parent or to learn vital information by looking at their 411. The disposable wristbands include the child’s first name, the parent’s first name and cell phone number and any allergies or special needs the child may have. These can be customized to fit any parent’s needs.