The nation's poor children and their families will lose a total of $5.2 billion in President Reagan's fiscal 1986 budget, said an advocate of children's rights who called the spending plan a "moral travesty." Marion Wright Edelman, president of the Children's Defense Fund, said at a Washington news conference: "More American children die each year from poverty than from traffic fatalities and suicide combined. Yet for the fifth consecutive year . . . President Reagan has targeted poor children and families again for billions in new budget cuts," she said.