The North American Free Trade Agreement is causing serious splits among Democrats. But the controversial pact is also putting some prominent political families under strain. Ross Perot, of course, is a leading opponent of NAFTA. But son Ross Jr. manages a Fort Worth airport that stands to benefit if the agreement is approved. He admits to having a difference of opinion with Dad. Meanwhile, Representative Rosa L. DeLauro (D-Conn.) has joined the opposition, while husband Stanley Greenberg's polling will help President Clinton in the fight for ratification.