Highlights wasn't only sold to
dentists and
pediatricians, as a deleted writeup suggests; I had a
subscription to it when I was about 6 or 7 (circa (1979-80). But I probably got the info on how to subscribe from a copy in some
dentist's waiting room. It's only sold by subscription, not in stores.
The company describes itself as "Highlights for Children is a general interest non-sectarian educational magazine for children aged preschool to preteen with a circulation of between 2 and 2.5 million copies per month and maybe twice that readership." The magazine has been around since 1946, and since then has expanded to have a catalog of merchandise related to or approved by the magazine and other enterprises, but there is no outside advertising in the magazine.
Sources: http://www.highlights.com,
http://www.ftc.gov/privacy/comments/highlights.htm