www.nytimes.com It is not very easy for children and young people to protect themselves from online predators and intruders. But, said Rosamund Coles, author of the paper “Choosing a Teen: Your Guide to Choosing a New Family and Growing Up Online,” if parents have a clear idea of how and where their children use the internet and let them choose their own digital friends, they are unlikely to encounter harassment or even strangers in ways that would keep them from going on to Facebook, Instagram or Pinterest. Coles, an associate professor of educational psychology at the University of Nottingham, England, and a mother of three, wrote “Choosing a Teen” as part of the 25th anniversary edition of “Guide to Parenting in the 21st Century,” the book by American psychologist Judith Rayner whose original material remains a standard reference for children and young adults living in what is now the digital world. Along with the contents of a wide variety of books, “Guide to Parenting in the 21st Century” has been developed with professional input from experts in parenting, the young, children and family counseling and occupational therapy. It is published by PublicAffairs, a division of Hachette Book Group.