Judging by the frequent sight of youngsters grabbing for Mom’s smartphone at the table and toting Dad’s tablet on car trips, it’s a safe assumption that touch screens have captured the attention of many children. But for parents (or grandparents) overwhelmed by the abundance of age-appropriate software, it helps to look for familiar faces in the crowd. Several internationally popular 20th-century children’s book characters have expanded beyond the confines of the printed page into the world of apps.