It was a little girl's idea of bliss and she had a ball. Toy manufacturers were having a convention at the Hotel New Yorker where Mr. and Mrs. William Rhoads and their children, Kevin and Kathy, were guests several nights. Toys were all over the place, and Kathy, who was in New York for the first time, was delighted by the displays. Kevin, who was making his second trip to the city, was interested in visiting museums, the planetarium, the United Nations building, and the Empire State building.