I think that kids playing outside with a little passive supervision (an open window) is fine at a certain age, probably around 10. Especially if there's more than one, and they know about streets and strangers etc.
I also think that cops stopping and talking to them in a non-threatening manner is absolutely appropriate. That's why we pay their salaries! I haven't had the negative experiences with cops that most of you have, perhaps because I look like a law abiding citizen.
Airlines are another story, I'm convinced a red flag goes up on registered democrats and that's why I get the lower-your-pants-a-bit-so-we-can-check-for-weapons-in-your-belly-button routine. But that's for another thread.
If a cop stopped to talk to my child, I'd make my presence known and ask him to tell my son how he can become a police officer to when he grows up. He wants to talk to my kid, he can entertain us for a while! Then again, I'm a little blasé, since my sun is only 6 months old, and we don't leave him in a room by himself, let alone outdoors. If a cop scared
my kid, that'd be another story altogether.
I have had a couple incidents driving around with my wife, because we are a mixed couple and that's frowned upon in our part of Southern California. However cops are just people, they are no more or less open minded than the society they are hired in.