Years ago on one of the current affairs programs they had something along the lines of this for leaving with a stranger:

Give you child a secret word that only you and they know. If someone says "mum and dad asked me to pick you up from school today etc' the child needs to ask what the secret word is. If they can't tell them they don't go - probably easy for them to pick a child up and walk off but still worth considering. Also easier to explain to an older child.