Matching pro tennis players with celebrities can make good tennis but usually better theater. Which may explain why a record 13,000 fans packed the West Side Tennis Club in Forest Hills, N.Y. for the third annual Robert F. Kennedy Pro-Celebrity Tennis Tournament. The biggest attraction, of course, was the Kennedys themselves. While the younger ones roamed through the autograph-hungry crowd, some of the older Kennedys took to the courts to the cheers of the crowd which brought in about $150,000 in proceeds for disadvantaged children. The high point for onlookers was when young John Kennedy wandered by mistake into the press tent. In escaping the horde of would-be interviewers, he proved as fleet of foot as any pro.