When he is down, kick for the head and groin. Avoid cops. Play it cool. There are not many rules in the primer for gang kids, but they all count. They are all easily understood, because they use a simple and sound philosophy--it's a stinking life, so get your kicks while you can. The gang is home, take what you want, tell them nothing--and do not get caught.   Two gangs of juvenile delinquents run riot in New York City. They constantly try to outdo each other with their clothes, weapons, language, and lack of morals. They are not just kids playing at war--they mean business. The only person who can infiltrate the gang is someone they can trust, someone like themselves. Someone who knows how to handle a knife and a gun . . .   If all you know of Harlan Ellison is his speculative fiction, prepare yourself for the breakneck reality of Children of the Streets.