a stranger's just a friend you haven't met

I believe a phrase like this would be better to teach kids, though of course it should come with the precaution that in fact a stranger is someone we don't know, and who we should be careful around until we know what they're like.

With sensible advice, like staying out of arm's reach of cars, not accepting a ride from someone you don't know well and trust, travelling in groups, etc., a child can learn to be cautious but still friendly.

In fact, considering reports that the "stranger danger" talks don't work well, because children frequently end up with the wrong idea of a "stranger" (basically they end up avoiding obviously scary people, which they would have done anyway), I believe such an approach would be more effective.

"Code phrases" can work well, if the phrase is given privately, the child can keep it to themself, and the phrase is changed after each use.

In any case, with modern communications there is little reason to need code phrases any more. Get the kid a cell phone, and keep them informed. (You can get a kid's cell phone that will only call home.)