Geat idea! Kids could wear them to parties, on day trips etc where the parents aren't present to ensure no one gives them the food they are allergic too.
They have transfers for No Nuts, No Shellfish and other allergies.
http://www.amazon.co.uk/12-Personalised-Rocket-Temporary-Tattoos/dp/B003BF2ADG/ref=sr_1_2?ie=UTF8&m=A28U598LQQ8KSW&s=generic&qid=1274735339&sr=1-2