Mimi Kayden, who was marketing director for Dutton, Penguin USA and Penguin Putnam for 28 years, and who left the company in 1999, has been hired as a part-time consultant by HarperCollins Children's Books. She will be taking over the responsibilities formerly held by William C. Morris, longtime director of library promotion, who died last September. In her new position Kayden has come full circle, as she began her career in children's books at Harper, working for Ursula Nordstrom.