There are two exceptions:
1) If you are the completely-innocent witness to a crime, you can talk to the police. But if you are not 100% confident that you are completely-innocent or the questioning strays from the crime you witnessed, stop.
2) If you did something suspicious that has a perfectly innocent explanation, you can talk to the police. (The classic example is if you borrowed your girlfriend's car, accidentally locked the keys in the car with the window slightly rolled down, and a cop pulls up as you are trying to fish open to door with a coat hanger.)
Otherwise, see the subject line.
Also, talk to your kids about how not to talk to police. Talk to your kids about not answering things that aren't questions. (For example, "If you don't let us search you, we'll have to get a warrant.") Role play with your kids.