Perky Priscilla Nichols, 6-year-old daughter of Mr. and Mrs. Bruce D. Nichols of Fairfield, Conn., tried hard to smother a big grin when she won her first big prize in her first big horse show but some of Priscilla's delight popped out in spite of her. Priscilla, riding Silver Hills, a 22-year-old pony, won the lead rein competition in the Fairfield County Hunt Club's junior horse show. A first-grade student at the Bolton School in Westport, she has been riding for a year and recently graduated from ponies to horses. After receiving a silver dish from Mrs. Gerald Vosburg, Priscilla told her mother, "I'll have my cereal out of it the first morning, and then I'll let you have it for cigarettes."