I always told my children there are good people and bad people out there. And the bad can be very tricky, even convincing us they are good. And to never go with anybody unless they know our secret word. I also told my kids not to tell anybody our secret word! That I am the only one that can tell someone. Then my kids would have a question if some one had our secret word, which would reveal an answer only we would know.
Gotta protect our bAbies! They are so innocent!
In hindsight, I'm shocked at the ease with which I collected my friends children from school on several occasions, I would get a late phone call saying my friend was held up at work, could I collect the kids and take them with me for about half an hour, this was a regular occurrence with 3 or 4 friends. On the odd occasion I was late I rang the school to let them know that someone else would be picking them up, I was hoping they'd relay the message to my daughter, but they never seemed to take that much notice.
I was well known to the children, but I'd just wait at the gate and wave at them, and tell them their mum would be a bit late so come with me. Even at the school that my daughter did not attend, so I was not known to staff, I was never challenged.