I would suggest not ruling out a general all discipline magazine. When you do you are really narrowing down your audience and a majority of Jrs. Are not so specialized that they do not want to learn more about other disciplines.
My 10-year old daughter gets Young Rider and loves it. Her only complaint is that it only comes every other month. She will read that thing cover to cover and they feature a large variety of articles from health to grooming to groundwork to western to dressage - well, you get the picture. Anyway, what she likes about it is that the content is easy to read (not too technical or dry) and she always learns something - even about a discipline that she doesn't ride.
My daughters passion is growing into dressage and we just started getting Dressage Today (school magazine drive) and she enjoys reading that too. Some of the articles in that are over her head though. She isn't quite ready to learn tips to perfect piaffes yet!!
Good luck if you decide to pursue this!